You can set a block on the minimum amount of characters required to be entered before the customer can proceed to add to cart / generate a quote when creating a sign.
The ability to change the minimum amount of characters is available for all letter models; this article, however, only refers to how to change it for the Advanced letter model.
How to set the minimum amount of characters on the advanced model.
Navigate to the Sign Customiser app for your store.
Click on Customisers in the home menu
Click on the customiser you wish to edit
Click on sizes in the left-hand side menu
Scroll to the bottom until you see the "Minimum Text Character"
Change the value to your desired amount of minimum characters that are required before the sign can be added to the cart or quoted.
Click save at the bottom of the form.