30X30 is the absolute smallest I would consider using. I prefer 32x32 or would consider 32x34.
Professional Choice makes some good pads but I prefer a 100% 1" natural wool contour pad. The wool does the best job for wicking moisture and cooling my horse's back and it conforms the best of all the pads. I've used the type with "memory foam" but it just doesn't do the job that wool does.
I do really like the Pro Choice brand. (As you know, I work in a tack shop, and see a lot of stuff come through.)
HOWEVER... I'd imagine that the wool back would become compressed fairly easily if it wasn't cared for properly.
I personally prefer a memory foam pad.
Anyways, they retail for quite a hefty price, I'll double check next time I'm at the store.
JDI, my wool pad has over 1,000 hours under my western saddle with no loss of effectiveness. The only care I give it is once per month I spritz it with water and brush it clean. I do give it a light brushing inbetween spritzing just before I ride. - love that pad!
I love wool pads too. The problem I have found with memory foam, at least here in Texas where it gets hot and humid, is that it really holds heat more and the horses back gets hotter than with wool. It may not be so bad if you live somewhere where it is cooler though.