I seems that your Garter Snake is big enough to take whole pink mice, but don't forget the importance of variety in your snakes diet. You could vary his diet with various fish and occasional worms. Take a look at our Garter Snake Care Sheets.
http://www.thamnophis.com/caresheets...ake_Care_Sheet
You'll find an abundance of helpful information in there. And what you can't find, just ask in the forum. Welcome here, and enjoy yourself.