Leave them in a mug of very hot water fr a few minutes. They will soften a little so you can open them up.

Buy a large pick.
My favorite thumbpick is the Dunlop X Large "tortoise.
Lots of choices out there.
I change the fit by dipping just the tight bend of the thumbpick in microwaved hot water to make it tighter or looser.

Welcome aboard, Ned.

Your best bet would be to visit a music store that carries stringed instruments, if there's one within reasonable distance of you.

Picks are VERY personal things, and there have been endless debates here on the Hangout about what's "best." In fact, there is no best - there is simply what's best for you, personally.

Let's separate out the thumb and finger picks, because they're different. For bluegrass banjo, your finger picks should be metal, not plastic. The good news is that most are very easy to adjust for size - you can simply open up the bands a bit. I generally don't make recommendations on things as personal as picks, but as a teacher I do happen to think that the Pro-Pik split wrap picks are an excellent choice for beginners, because they're the easiest to adjust for comfort and retention on your fingertips. That link will show several different blade styles... I wouldn't worry about that for now.

Now, as to thumb picks: again, a good music store should be able to set you up. There's an almost baffling array of shapes, but you're correct that sizing is really important. For the record, most plastic thumb picks are made of thermoplastic and a (very brief!) dip into boiling water may soften them up enough to easily shape to your thumb. But probably better to get one sized properly in the first place.

If you don't have access to a suitable music store and need some guidance on online sources, you have but to ask.

I’ve molded plenty of mouth pieces for football, fighting, by putting in very hot water. There’s reason it wouldn’t work on a thumb pick. I’ve never gave it much thought. Go to banjo Ben online store. They have a variety of size and shapes for just a couple of bucks. Welcome to the hangout and good luck.

There’s no reason it wouldn’t work. By placing in warm water

I like microwaving my cup of water for 2 minutes.

I was a Dunlop guy for years, including their metal thumb pick. The plastic ones just never worked for me. I also tried those very pointy Earnie Ball picks, but went back to the Dunlop’s with a JD Crowe Blue Chip thumb pick. Those are definitely worth the $40. It’s metal, so it doesn’t slip or get caught in the strings, but it has a plastic tip. Finally, I found Ross Nickerson’s cobalt plated “Nick Picks” on his website. That’s all I use now. Head strikes are now eliminated and my speed and accuracy have increased by quite a lot. Don’t be cheap on picks.

Just don't make the mistake of heating a plastic pick with an open flame! The results are not good!!

I still use a Dunlop metal thumb pick. The trick is to smooth out the tip with a file, and polish it. I tried Pro picks from Deering for fingerpicks, and it’s what I’ve been using ever since.

Just buy a large thumb pick, they are cheap (usually). You can make the medium fit, using techniques described above, but the whole pick is still a different size and you won't be happy with it down the road.

I get the Dunlop xl tortoise thumbpicks.
I then shorten and round the business end,removing about 3/16" off the tip,rounding with a file and smoothing on printer paper.
I sometimes recurve the band to fully fit the thumb all the way around.
By dipping only the part of the pick that needs changing the rest of the pick is cool to hold and stable.