Much of stall size consideration is based on the size of your minis ---- and how much they will be in the stall.
I have several 6X8 and for my 27-28" minis they are wonderful. But, not for days on end!!! For the B size there are 8X10 or 8X12, etc......useful if a small one needs to be kept in for illness, etc., also. Mine are usually out in the fields with run-ins. But foaling, conditioning, etc., I use a stall. Mostly just overnight.
Be sure they can move easily, lay & roll without issue, have a "far end" for elimination and another area for food/water.
Also, if the stalls are permanent, think about resale value. Several of mine have a removeable center so I could have larger size within 10 minutes. The entire barn is build so that the stalls could be totally removed and re-mapped with no issue as to the soundness of the main structure....even totally left out and a huge open inside would be available. I put in foundation, blocked up, framed on that and trussed the structure. I do not have a loft. Have a separate hay building.