rate of use depends on size of critter, breed, (easy keeper old stock appy vs. warm blood) how active it is, (pet vs. working animal) and manners. (does it pull all the hay out, spread it into a finely trampled mass and whizz all over it, or does it eat quietly at the feeder and not waste a lot?)
4x4 bales of brome or orchard grass run about 800 pounds.
4x5, 5x5, 5x6, and 6x6 bales get progressively heavier up to a ton.
white mold isn't really going to make a gelding sick, so long as there's a way for it to air out as it's flaked off, as long as there's not lots of it in there -- but if you're feeding it to a mare, it can cause her to miscarry.
the mold comes from baling the hay too wet. (still damp from dew, not raked, then turned and fluffed again, etc...)
when feeding small bales, I generally estimate 1/2 bale per horse per day for non-working animals - for an average of 50 pound bales. This covers variations in weather, since they'll eat more when it's really cold.
with free feeding big bales, expect to lose 1/3 - 1/2 of the hay to waste, unless you're in there twice a day forking it back into the feeder before the idiots trample/whizz it into mulch.
Even with a feeder, in the past I've forked it full each day instead of leaving the whole bale in it.
what kind of horses?
quarters, warm bloods, old stock appys?
sorry if it sounds like I'm lecturing - and telling you lots of stuff you already know.