10 x 10 would be adequate, larger obviously more comfortable, but not necessary.
Just as a comparison, a 12 x 12 in some places here would be considered a waste of space, where I bought one of my mares from the stables were 9 x 9 and housed full grown quarter horses 24/7, some of them stallions. When my mare was delivered, the Trainer eyed up my (to my English eyes small 10 x 11 stables) and told me that if I knocked them all down, made them narrower, I could get an extra 2 stables in the barn.
I was reading a question similar to this on another site and it didn't shock people at all that 17h horses were kept in 11x11 stables, and smaller. I can imagine the reaction it would get here.
17.3 has a 16ft x 14ft stable
17hh has 12ft x 14ft
14.1 and 14hh share a 13ft 6in x 20 ft, which has a half wall down the middle so they can groom each other and pinch each other's hay over the top
9hh has 9ft by 14ft
8hh has 9ft by 13ft
They're all odd size stables, but they all seem fine in them. I think people forget that a lot of horses are still in stalls.
15' x 15' stables, the biggest i've ever had. they're fantastic and the horses love them. when i put them in the one small one (14' x 12') they kick the hell out of the walls...
16.3, 18h, and 15.3.
I have one 25' x 15', which I hate TBH, it's far too big! It will now be used as an isolation box for the livery yard. The stables for liveries are all 12' x 12' and my own five 12' x 12'. I'll never have a horse bigger than 16.2 myself anyway.
I used tyo have two 10' x 10' stables for Bramble and Teazle who are 13.2
My stables are 15ft x 12ft (17.1hh) and 14ft x 12ft (16.1hh) and 12ft x 11ftish (16.3hh). They are all adequate spacewise and the 12 x11 is not 'officially' a stable! (OH kept putting machines in it, to stop me putting a horse in!!!)