This is the only thing I could find that had an actual recommended catheter size:
www.uihealthcare.com/depts/med/urology/research/bladdercancer/bcginterferonalphastudy.pdf
It's from 1999 though so who knows if they've come up with some standard that is different.
It says recommended size for men is 16 French and for women 14 French. (If you go to the link, you can do CTRL F and then put in "Recommended catheter" and it will take you right to the section.
You can ask your doctor about the possibility and pros/cons of using a smaller one if you find it causes a lot of pain each time (even after getting a relaxant). Who knows? Could be some docs like to use the bigger catheter because it's faster.
Like doc says, nobody really knows why some people have a lot of pain and others not so much.
As for me, now that I have a neobladder and no prostate, I can catheterize myself with a 16 French, no numbing stuff, no pain, not really any discomfort and I can do it practically using a windup.
However, I do not recommend getting your prostate removed just to make catheterizing easier.
That's a joke!
Again, good luck with your treatments and I hope the extra time with the numbing gel and the relaxant helps.
There are some other things in the article that you may find of interest and you may even want to ask your uro about. It's meant for urologists and it's specifically talking about BCG with Interferon administration but that is likely not much different from regular BCG in terms of how administered.
Mike