Hi Bob
There are no rules around how big BUAs should be. These can run from the very small to the huge (whole towns).
On your practical point - I guess how big you want to make yours depends on the scale of models you use.
We play in 6mm - and model bases representing sections of BUA tend to be no bigger than 10cm x 10cm (often smaller)..just to make things practical. Larger BUAs can then be built up from putting sections together.
In our games the buildings do not have to be occupied for units to be counted as in the BUA - other people do different things - this is entirely up to you.
Sounds like you have something in mind - and want to give cover to infantry in the hut ...so that would work fine.
Number of units that can fit in a BUA? - We play as many as will fit in the area...and dont play various floors of buildings (e.g. no stacking).
Cheers
Adam