The US Bishops offer their guidance on the size of the altar:
§ 58 § Although there is no specified size or shape for an altar, it should be in proportion to the church. The shape and size should reflect the nature of the altar as the place of sacrifice and the table around which Christ gathers the community to nourish them. In considering the dimensions of the altar, parishes will also want to insure that the other major furnishings in the sanctuary are in harmony and proportion to the altar. The mensa should be large enough to accommodate the priest celebrant, the deacon, and the acolytes who minister there and should be able to hold The Sacramentary [The Roman Missal] and the vessels with the bread and wine. Impact and focal quality are not only related to placement, size, or shape, but also especially to the quality of the altar’s design and worthiness of its construction. The altar should be centrally located in the sanctuary and the center of attention in the church.
In many churches the matter of square (or round) and rectangular is an issue. More churches opt for square these days. In my parish, the Liturgy of the Eucharist could be led from any of the four sides. Square permits a central location for the elements–an important sacramental emphasis that dodges the pre-conciliar “meal settings” of the clergy on the long altar.
I remember a few churches which attempted to place altar and ambo in the sacntuary, not unlike two sides of a balance. Not a bad idea, but I do like the central altar a bit more. People accustomed to thinking ot viewing the altar from the nave should note that a “central” location includes the center of a space from the back wall of the apse to the front border of the sanctuary. This “center of attention” also puts to rest somewhat the reform2 “need” to advocate for the priest turning his back to celebrate toward the east.
Comments?
All texts from Built of Living Stones are copyright © 2000, United States Conference of Catholic Bishops, Inc. All rights reserved. Used with permission.
Catholic Sensibility 
My collegiate parish church (which had burned down in the late 1970s and was reopened my second month of attending college; and was later converted into a parish hall when a new church was built next door 1990, and then itself replaced by an entirely new parish hall a few years ago, it seems…and the Dominicans are now building a new priory across the street…) had a concave triangular altar (with flattened points) of dark charcoal slate (the pedestal, too); IIRC, the stone was a bit shy of a foot thick, but my memory is not that clear on that point. The church space was a convex triangle, and there was a deep aperture to a skylight above the altar on a predella in the center; the reason it was deep was that the ceiling got lower from the edges towards the altar area. The ambo was along the back wall, axially placed; the tabernacle at one end of that wall and the font at the other.
PS: Now I am remembering something very different, but perhaps a very distant relationship in terms of inspiration. One of the greatest university chapels ever built was the one Borromini designed for the Sapienza in Rome: it’s a triangle. With one fabulous complex triangular/hexagonal dome:
http://www.flickr.com/photos/dikele/4522591627/lightbox/
It’s a tricky place to get into if you are not attending the university there: but it is open to the public for Sunday Mass, and that’s how I saw it. The space is luminous and warm acoustically (because, even though it’s a somewhat deep dome, it’s fitted precisely to the space, so the acoustic works).
Now I wonder if the idea for the much less glorious space we had in college was partly inspired by this much more famous exemplar of a university chapel. Bonus image of the courtyard and famous spiral spire that tops the dome: