The laws for handicap parking
signs are federal, and are in the ADA. Here's a fact sheet about handicap parking ADA compliance:
https://adata.org/factsheet/parking
There is no letter size requirement there, but the general ADA rule on lettering
signs is contrasting colors, plain block letters. I checked our state law, and it's more general than the ADA rule, so the ADA rule would be our guide here.
The biggest mistake I see made in marking handicap parking is the location of the
sign. It has to be right at the end of the parking space, at 5' above the ground or more. Basically so that it's visible to a driver, right in their windshield view, and cannot be missed as the pull or back in. And each space has to be marked.