You can set the maximum width and maximum height allowed for a sign to be created and added to cart / quoted.
When these limits are met, a modal will appear warning the user, and the ability to add to cart/quote will be blocked.
Changing the sign size dimension restriction is only available for the "Advanced" letter model customisers.
How to change the sign size dimension limits
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 "Maximum sign width" and "Maximum sign height" options.
You can set a width or height limit by changing either option from "Unlimited width" and "Unlimited Height" to selecting either "Width limit" and "Height limit".
On each associated size dimension you can then set the maximum value that is allowed for that dimension by changing the value to your desired length.
Click Save