Design your bow, build your form, then you will know how big of a block you will need.
I buy my blocks in mostly 2" x 2" sizes. I buy in lengths of 18" for longbows and 24" lengths for recurves. 18" stuff is much easier to get for me and about 1/3rd the price. Specify that you do not want any wood that is completely covered in wax, ends are okay. Some places saw the wood green and dip it in wax right away. They don't want it checking on their end but within a week of when you remove the wax it will check badly. I won't accept any wood that is over 12 percent moisture content or it will check in my dry shop, especially in the winter.
Any company that deals with wood will have a moisture meter and you should have one too.
Good Luck, Chad