Is My Milk Snake's Growth Stunted?

[blackbear]

Hi there! I've had Paprika, my Sinaloan milk since Feb. of 2014. I believe she's female. I never had her sexed, but the tail tapers evenly so I've just been assuming. I bought her at a Petsmart, which we all know takes great care of their animals... When I bought her, they told me she was already at least a year old. She was about 18-19 inches long at the time, if I remember right. I've had her in a 20 gallon tank all this time with aspen bedding and multiple hides (the cardboard tube is her favorite), and as of 5/7/17 she's 29 inches long. But she's still pretty skinny. Only the width of my middle finger at her largest part (a smidgen less than 3/4th of an inch). She's been eating well and has had proper tank conditions these past three years, with only a few times not having a proper shed. She sheds anywhere from every 3 weeks to every 5 weeks. She's currently being offered 3 peach fuzzies, and today is the first day she's accepted the 3rd one.

Is she a normal size for her age? She is at least 3, if Petsmart was wrong about her already being at least 1 at the time of purchase. What size should she be, girth-wise? I've been searching everywhere, but the only size info I can find is length, and she doesn't seem to be full-grown yet, despite being 3+. Could her growth have been stunted from being in a 5 gallon tank for at least one year (and possibly not properly fed)?