Trouble setting max characters for form fields

This should be simple, but I am just familiarizing myself with HikaShop, and can't seem to figure this out. What I need to do is limit the number of input characters for select form fields...specifically the postal/zip code and phone number fields. For some reason these allow the user to input too many characters. I've been poking around, but all of the basic things I've tried don't work. Where should I be changing the limit of characters for these fields on the contact and shipping form? Thanks.