My list of Tub sizes:
6 qt - for hatchlings
16 qt - 500 or less grams
28/32 qt - 2000 or less grams
41 qt - 2000 + gram adults
You want the heat pad to cover around 1/3 of the tub.
So the small size of petstore heat mats should be fine for a hatchling, but be prepared to upgrade soon as babies grow pretty fast.
Buying heat tape is cheaper.