Where are you? Are you looking to live cover or AI? That will make a big difference on your possible stallion selection.
What you should be looking for, is a pony that is true to the Welsh B breed standard, and isn't going to risk throwing an over-height foal. If your mare is close to height, you would be better off breeding to a smaller stallion. If she's well under height, it doesn't matter whether you use a tiny stallion or one close to the maximum height of 13.2hh.
I really like this little guy, Welsh Pony stallion at stud - *Wedderlie Mardi Gras LOM/AOE - Imported Section B
There's a STUNNING stallion based here in Western Australia called Penley Naturalist, he is youngish but spectacular.
Whichever stallion you choose should be well put together and a good representative of his breed, so you should read up on the breed standard so that you know what to look for.