It should be large enough for the snake to soak if it needs to. Also a large water dish helps keep the humidity up! A 5 inch pot sounds a little to small for an adult. It should at least the same size as their hides!
Only my smaller snakes <300 grams can actually fit in their water bowls. I'm lucky enough to have like 4 different sizes, but the biggest ones are still only 5'' across and i use them for even my big girls.
No, the bowl doesn't have to be big enough to soak in. Soaking is not an integral part of a BP's life. They need fresh water to drink, and that's it. Nothing wrong with providing room to soak if you wish to....but it's not necessary.