Water Dish Size

How big should a water dish for a snake around 300grams be?

A 3" diameter dish would be fine.

I use 8 oz. deli dishes until the snake starts turning them over. Once they do that, I switch to a 4" PVC coupler with a 16 oz. deli cup.

All my snakes have 5" or 6" crock dishes.

I match the size of the dish to the amount of humidity I need in the tub. Size of the snake has nothing to do with it.

I'd agree with slim. If your humidity is low you need a larger diameter (depth is just how often you fill it...) if your humidity is high smaller. I use between 2 1/2 inch diameter and 10 inch depending on the enclosure.