Hi,
I have a lot of experience with turtles and tortoises, they're my main hobby.
For one painted turtle, I would start in a 10. Move to a 40 gallon breeder. Once he's big enough to move out of that tank, I would suggest building a pond. Its much easier and cheaper then you would think. What I have done is just dig a whole and buy play sand. Home depot sells it in 50 lb bags for $4. A large pond would cost maybe $30 to cover in. Needless to say, they are not hard to care for, and very interesting. For feeding, simply use river shrimp/red shrimp and pellets (doesn't really matter what kind, go for one of the more well-known brands.)