Our current ones are all weird sizes due to being ancient farm buildings, the stallion has one that's 14 x 10!
The new ones will be 5 that are 14 x 12 and one that's 14 x 16.
When we eventually put more on the opposite side they are likely to be bigger as I like them to have a secondary use as mare and foal boxes.
Gosh I don't know what size Tobago's box is - but it's huge; he can (and does) canter round it! Used to be foaling/mare-and-foal boxes on his yard, which was a TB stud, so they are all enormous. He is a 15.1 Arab stally and looks tiny in there - and the doors are very high...