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.
This is a good website for wool pads but Reinsman also makes a nice one for under $100 I think.: Saddle Pads & Blankets, Wool Saddle Pads & Blankets: 5 Star Saddle Pads
This is an excellent article about pads: Saddle Pads and What Manufactuers Don't Tell You!