Hi Kiersten,
Right now it's not possible to limit the amount of characters in a free text field.
I did take a look and there are a few ideas in the Innovation Lab about this, so depending on where you want to use it, etc, maybe check out some of these to consider adding a vote or two?
Definable character limits for custom fields (this one is getting close to the threshold too.)
Limiting the amount of characters in custom fields
Kyna