There are three common sizes, you could say small, medium and large !
But then there are some less common sizes as well which you might encounter.
Worse still, its roughly specified as the weight per foot length !
I once had to sort out a problem for this fellow which was mostly of his own making!
He had a leaking joint between the lead incoming and his copper pipe. Contractors were doing some Thames Water work in his street so he called them. He gave them £50 and they fitted this "big thing" and it solved the problem!
Well for a few days until they had moved on. Then it started leaking about 4 li/minute.
I took it all apart and could not work out exactly what they had done wrong but to be sure I replaced it with a lead loc !