Better signs are a calculator (10-15% of the snakes weight) and comparing the mouse to the thickest part of the snake's body. The mouse should be as thick as the snake's thickest part.
If the pinky feels too small but the snake isn't yet ready to move up for a bigger food, it's good to give two pinkies. First you can give him two pinkies every other feeding time (and then keep a bit longer rest for digestion) and one pinky every other time. Then you can move up to two pinkies every time, and keep that going untill he's redy to move up.-