The main criterion is "big" LOL Seriously.
As for whether bigger coops should be square-ish or long and narrow, it depends. If you are in a very cold-winter climate then squareish is valuable. If you want to have a jillion different runs (lots of separate pens, each with own run) then long and narrow is better.
In any design, if you need space for pairs or trios to be put temporarily for matings, it is not hard to build wall cages or shelf-type pens. Although it is much much harder and more annoying to try to make attached run space for these.
Depending on what your hatching schedule is, it can be useful in serious-winter climates to have grow-out accommodations for summer use only, that way you are not spending money on winterworthy space for more chickens than you will actually overwinter.
Good luck, have fun,
Pat