Hello and welcome to the forum. A snake from Greg will be a good snake.
What species are you getting?
I had to laugh at something you said. You can't get away with keeping whole (blood and guts still inside) pinkies in your parent's freezer but you can keep the same pinkies cut up in an ice cube tray. Imagine their surprise when they get some ice for a glass of ice water
Just messing with you.
Sounds like you are doing some research before the awesome snake arrives. That's great. You've probably seen the forum care sheet but I will leave a link anyway. Feel free to ask anymore questions you might have.

To answer your question about space, I think choosing an enclosure that is manageable for you is best. Babies tend to burrow so finding the little scrub will be important especially when you panic thinking it has escaped Can't say I've ever heard of a garter not eating because of too much space. The 18 X 18 sounds like it will work for awhile.

Garter Snake Forum - Garter Caresheet