Appropriate feeding size? (adult mice)

I've got a hatchling coming tomorrow and I'm curious at what point can/should I try adult mice if at all? I feed them to my Corn so it would be nice to only have one size that fits all so to speak! I do imagine he will eventually need larger feed, and for now probably smaller? Is it entirely based on the largest point of the snake's girth, once that is equal to the mouse size?

10-15% of his body weight. Probably about 10-15 gram mice or rats. Adult mice are about 25 grams.

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Wow that's such a small percent but I guess that makes sense, I'll have to pick up a scale. Any suggestions?

Just try your local Walmart. Find a digital kitchen scale that weighs in grams, etc.

If not amazon has them.

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Another good rule of thumb is that you want the girth of the food to be about the same as the girth of the animal.

Also i picked up my scale at kohls for 20 bucks it goes up to 7kg.