Hello

,
Gowan and msgreg are both correct, size depends entirely on your own preferences and how you like the feel of the stones in your hand.
Mr. Kuroki can offer you some sample stones of various sizes (at a decent price) if your interested, just send him a message.
For a cheap set - Size 24 under - These stones are cheap, easily replaced and very portable, but tend to chip or crack easier than larger sets.
For Portability - Sizes 25 - 30 are the best, they're smaller so don't require large bowls, but still durable and comfortable in the hand.
For Aesthetics - Size 32 is considered the most appealing, based on the standard board surface. However, sizes 31 and 33 would look about the same. There is only a .3 mm difference between sizes and you'll likely not notice it until you get sizes 2 or more apart.
For Professional Use - Size 36 is considered the norm. For professional games (at least in Japan) this is the standard stone size. As a side note, I like the feel of size 36 but that's based on my own personal preferences (still think 32 looks better though).
Size 40+ - These stones are big and very unwieldy, still, if you like the snap of sound on the board, there's nothing better. Unfortunately, you risk deep dents in the surface with these stones (depending on wood hardness).
Mr. Kuroki has only up to size 40 listed and he skips several unusual sizes between 1 and 30. He will likely have these sizes in stock, but unadvertised so you'll have to ask him about them. I know Mr. Bozulich at Kisedo has (or had) a couple standard grade size 44 and 48 stones for under $1000 (50 is the largest). I've seen these in person and they are HUGE!
Anyway, I hope this helps
