Cant really go by weight as a judge of if a snake is ok. Just physically look at it and make sure it looks 'healthy'. If the backbone is showing or ribs are poking out, then its obviously underweight. Just from looking at the first picture, he looks a little thin to me but that's from looking at my snakes which are kind of chubby as they like to eat and I like to feed them . As for the mouse being the right size, you can either weigh it and if its 10-15% of the snakes weight, then that's ok. Or you can just eyeball it and feed it a mouse that is about as thick as the thickest part of the snake. This only really applies to babies and juveniles as adults go on a maintenance schedule which is usually a small or medium rat once a week or 2 weeks depending how big your snake is.