The US bishops have a document called Built of Living Stones. See old.usccb.org/liturgy/livingstonesind.shtml
§ 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.