Hi Jake, and welcome to the forum.
It certainly sounds as though you've got yourself a Red Sided Garter (
Thamnophis sirtalis parietalis). You won't be able to guess the sex of the animal by size alone, at least until it is adult. Females are generally larger and more heavily built than males.
You should find much of the information you need to get you started in our Articles Section (centre tab on the home page). Also our Care Sheets might be of interest to you.
http://www.thamnophis.com/caresheets...ake_Care_Sheet
Enjoy it here!