I really don't know the dimensions of our round bales, but most have been weighing in at about 1200# (some heavier).
right now I'm feeding 12 minis off one round bale, and I think its lasting me about two weeks (just started feeding the whole group together recently due to incliment weather and having to rearrange corrals), might be a little less. [Those 12 consist of 3 yearlings, 6 mares, 2 geldings and 1 stallion; all B-size.] Before rearranging, I was feeding 6 mares and 3 yearlings off the round bales; they were lasting me about 3 weeks. The stallion was getting small squares (his stack is about gone, and currently I can't get through the snow to get to it); and the geldings were in their own pen and a round bale lasts them about 2 months. Oh, and I fork them hay twice daily; I don't know how long it would last if they had free choice. [Lately, the 3 saddle horses have been going through a round bale in about a week to 10 days; usually they last 2-3 weeks, not sure what's different right now.]