Clipped from: https://www.newliturgicalmovement.org/2010/10/on-size-of-altars.html

New Liturgical Movement: On the Size of Altars

1. Shape: An altar should be rectangular, never round or octagonal, and only square where space constraints require it. I have seen one round altar in my life, a obje done recently in an otherwise fairly competent classical style, though wholly inappropriate rubrically and theologically. Scripture speaks of the "horns"--the corners--of the altar, and the round altar carries a whiff of the occult to it. The octagon is appropriate to baptisteries, not chancels. The Old Covenant's altars were rectilinear, and as the altar represents Christ, Christ calls Himself the cornerstone.

Cram's altar-like communion table at East Liberty Presbyterian Church, Pittsburgh. Cram once commented all the interior required was six candlesticks and a crucifix to be ready for a pontifical liturgy. Note its extreme length. Source.

2. Proportions: Ralph Adams Cram, the great churchbuilder, is our best authority on this point, and in his book Churchbuilding makes much the same point about the rather underwhealming communion table which was at the time installed in the vast open chancel of Trinity Church, Boston, describing the empty sanctuary as "dead," the "black walnut table of small size" overshadowed in one photograph of the period by rather silly floral arrangements placed around the eagle lectern, itself infelicitously placed at the center of the chancel steps. Cram suggests that the church's principal altar should have a width of around one-third that of the nave; he suggests somewhere between 8 and 12 feet, the higher end of the scale being determined by the upper limit of altar height (3 feet 4 inches, in Cram's mind, a fairly comfortable number; J.B. O'Connell, about forty to fifty years later, suggests 3 feet 5 inches). These are not necessarily ritual requirements, but that of "art; which is also a question of religion, since art in the service of the Church, is simply art as an incentive to religious emotion." One may rightly question if it is only that, but Cram's point is well-taken.

A large altar set in a suitably spacious sanctuary; it could be improved slightly by the addition of candlesticks and perhaps even a hanging rood. Source.

3. Dimensions: Cram is, above, speaking primarily of altars intended as part of a larger composition including a reredos, which itself acts as something of a magnifier for an altar in a large church; his rule of thumb is a good one, but probably should be adjusted by sight for a freestanding altar with tester or baldachin. I am sure I have also seen reredoses with engaged altars that are longer than 8 feet, though perhaps the inevitable elongation is, as Cram points out, not particularly suitable. Probably about 12 feet would be the upper limit in most older churches. Most rubrical sources suggest that a bare minimum of 6 feet should be the starting point for length, if only because that length recalls the tomblike symbolism of altar's relic sepulchre. As a practical matter one could probably celebrate the old mass on a mensa of 3'-10" in width but it would have to be a very low (or narrow) mass indeed. I would be inclined to think that 6 feet be the bare minimum length, and seems rather small to me.

Preconciliar sources (such as J.B. O'Connell's Church Building and Furnishing suggest the mensa depth (not including a tabernacle or gradine) ought to be between 1'-9" to 1'-11" with a large altar, and four feet if one has a tabernacle with altar cross placed behind it. Cram suggests 2 feet even. Both of these seem to me rather impractical in a modern context. Most freestanding altars today will not (sadly) include a tabernacle directly on the mensa, but ought to be designed with space for a row of candlesticks and crucifix, so at least three to four feet seems like a safe starting-point. Given that it is easy for an altar to become overcrowded with ciboria at large masses, I would think one could perhaps even push it to five feet in depth if the sanctuary was large enough. A logical rule of thumb is to ensure that objects placed in the center can be comfortably moved without access to a stepstool from one or the other side of the altar, which would preclude anything over five-and-a-half or six feet in depth. Mocking up the altar first might be the most sensible option.

4. Placement: Most altars today are placed well forward of where they might have been located fifty or sixty years ago. This may well be a positive development in theory, but it has not been one in practice. The almost total abandonment of altar steps has also been exceedingly unfortunate, though that is a topic for another time. O'Connell suggests circulation space of at least 2'-6" between the back of an altar and the wall if it is to be consecrated (following the rubrics then in force in 1955) and that dimension is a useful one to consider when trying to determine the bare minimum of circulation space around an altar. Considering this probably does not even take into account assisting deacons, altar boys, and the rest, there should probably be considerably more space than that before one gets to the walls, sedilia, or other impediments. The space in front of the altar, beyond its raised steps also ought to be particularly deep if possible.

As I have said repeatedly in the past, most altars today are placed in sanctuaries not designed for them. The modern altar placed at the level of the chancel is occupying space originally intended for the graceful movements of the sacred ministers at high mass. Given that most freestanding altars will probably be used from both sides at some point in their lifetime, probably the space around it needs to be twice as deep as it usually is, and somewhat broader, given older altars usually did not require circulation space on the short (north and south) ends. Most sanctuaries today go the opposite route and seem to shorten the depth and widen the breadth to almost shelf-like proportions.

A good example to study are churches built in the era immediately before the Council, where some experimentation had begun with freestanding altars and versus populum liturgy but it had not become normative. Some churches placed the altar at the crossing (often an architecturally messy proposition) but still ensured there was enough space around it on the raised sanctuary platform to avoid it turning into a catwalk. On the whole, an enclosed sanctuary, even if it may not be as visible from the transepts, may result in a more satisfactory architectural solution.

The ciborium magnum of the National Shrine in Washington, D.C., gives a good impression of the amount of space required to properly accommodate a large baldachin. Source.

Another important matter to consider is whether there is to be a ciborium magnum or baldachin, which, when placed in a smaller sanctuary, can cause acute circulation problems both around the altar and around the entire structure. If there is not ample space around it, one might consider thinning down the baldachin's members--there are quite a few handsome examples of delicate metalwork ciboria--or simply adopting a hanging tester or canopy instead.

The altar, as Cram says in the quote above, is the font of the life of the church structure, and its apex and summation. It is not enough to simply apply some traditionalizing edging to a liturgical coffee table, but we instead must ensure that this cornerstone must fit with perfection into its surroundings as the noble site of our bloodless participation in Christ's sacrifice.