Is there a head to body ratio you look at? Does it vary a lot by species?

I've been feeding my Eastern Garter what I judge is an ample amount, but I'm inexperienced and want to stop before it's too late if I'm over feeding her.

I give her three (3) pinkie mice a week and sometimes give a few bits of fish fillet during the middle of the week. She's about two (2) feet long and maybe two (2) inches thick in the biggest areas. Think she's too big or moderate?

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Here is a recent photo.