The criteria is relative size of the prey to the snake. If a small/mid sized mouse diameter is 1.0 to 1.3 times the snakes body, it would be appropriate to move up. It should leave a noted bulge when your snake gets it down to its gut. If two fuzzies do that or just barely, go ahead and move up. It will take it a little longer to get it down, but don't worry that is normal.
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2.3 Albino Nelsoni Milksnakes
0.1 Snow Corn
0.2 Sunglow Corns
1.1 Albino Motley Corns
3.3 '04 Albino Corn hatchlings
1.0 '02 Albino Stripe Corn
0.0.12 Sunglow/Moltey eggs simmering
0.0.20 Snow/Amel-Het Anry eggs simmering
0.0.5 Sunglow/Amel-Het Anery eggs simmering
0.0.13 Motley/Stripe eggs simmering
Bunch.Bunch Bearded Dragons
Bunch.Bunch Crested Geckos
0.1 Great wife
2.0 Great boys
0.0.2 dogs (they're great too!!)
and Corey's Yellow Knee Tarantula "Fang"