i would say get the biggest barn you can reasonably afford. you
never hear anyone say they wish they had a smaller barn. if you only
want around 100 cages, then only put that many in it. then you have
the rest of the room for organizing and storing your carriers, nest
boxes, hay, feed - whatever. (and even anything non bunny you need a
place to store- lawnmowers and stuff).
i suggest a monitor style barn if you can, that genreally affords the
best for setting up cages, and the high roof in the middle will keep
the heat above the rabbits, and if you build it with a loft for
storage above, then it will usually keep the heat in a bit better too
because of the added insulation that brings with it.
my mom recently got one of these types of barns, and raises primarily
french lops, but also has a good number of mini lops, mini rex,
silver fox, and whatever else wanders in from time to time! she can
comfortably fit 100 or so holes (and that is with a very nice waste
disposal system for most of the barn.)
the waste disposal system is another consideration. if you do a row
of hanging cages and put a tarp underneath (she had one made
specifically for this, so its nice and heavy duty and the perfect
size). it tapers down to one end, and empties into a strainer to
keep hay and heavy debris out. and the liquids and smaller waste
goes into a septic system! how cool is that!
if i were to build the barn i wanted, with no restrictions that is
probably how id go to it.