UVB helps the reptile synthesize D3 which is used to help the body absorb calcium. Now there is the whole argument of how much D3 is absorbed when fed calcium with added D3 but that's a whole nother subject. Sure UVB light wont hurt a reptile unless you are using something crazy like a T5 HO at 4" and will be beneficial but since BPs pretty much never come out in the day, not sure how much it will help. My boas are out pretty much all the time so for them, a 5.0 UVB tube wouldn't hurt but I don't use them. I just take everyone out for about 15-30 mins of sun every couple days in the summer time.

As for food size, about the thickness of the thickest part of the snake give or take a little. Certain snakes are more tolerant than others. Pythons can eat huge food. BCIs can take pretty large foods. BCCs go smaller on as they are more prone to regurgitate if too large. Boas are more rigid so I wouldn't go too much larger than their thickness. Pythons, I've fed Dottie rats that made her look like she ate a football as a baby and she literally waddled into her hide and sat in there for like 3 days and then was back out ready for round 2 lol.