Ctom has messaged me about this, and I didn't realize someone else was asking, so sorry for the delayed reply.
I bought the 4 smallest sizes that High Temp Masking Company sold at the time, and the two smaller sizes have been the most useful, which would be 3/32" & 1/8". It's one of those things that you have to test to see which works the best for you, since all hooks in a size are not the same.
As long as the cap fits snug on the hook, where it won't come off easily, that's all you need.
If the cap barely fits snug, and you heat them, they tend to fall off, so what I now often do is put the cap on the hook eye as I take the jig from my oven for coating. I've learned to do that quickly enough that the jigs still hot enough for coating. Then I remove the cap and reuse it. Sometimes, dry powder will stick to the cap, but easily comes off. You can also place the caps in the oven and heat the paint and then it peels right off.
I see now that they have smaller sizes, so I would get some of every size from 1/4" down to the smallest size .020" size for the jigs I pour. That's a worthwhile investment IMO.
For the larger size jigs typically used for bass or saltwater, 1/8", 3/16" & 1/4" should cover most hook eyes. If you use Gamakatsu hooks, which tend to have a smaller hook eye, the 1/8" should work. I use jig hooks from size 12 up to 10/0, so those caps have been very useful.