1. yep, feed a rodent about as big as the widest part of the snake's body. increase size as the snake grows.

2. like Argentra said, don't worry as long as your temps are right.

3. you can't tell age, but you can tell life stage. if it's a couple feet long, it's obviously still a youngin. anywhere from 3 to 5 feet could be full grown, depending on whether it's a male or female.