There isn't currently a way to limit the character count within a custom field, however, you can create a calculated custom field that pulls in your original custom field to give the user a character count. You can then add a third custom field, that says something like "If calculated field > 20, please update custom field to less than 20 characters."
It would look something like this:
- Original custom field name = Custom ID
- Calculated custom field #1 name = Character Count - Custom ID
- Calculated custom field #1 calculation = LEN({DE:Custom ID})
- Calculated custom field #2 name = Character Count
- Calculated custom field #2 calculation = IF({DE:Character Count - Custom ID}>20,"Please update Custom ID to be less than 20 characters")