The minimum size of the uppercase and lowercase characters of a font that are used in sign design determine the minimum height of the sign size.
Using the advanced model you can set the minimum height of the sign size by doing the following.
Navigate to the admin of Sign Customiser on your store
Click on "Customisers" in the home menu
Click on the customiser you wish to edit
Click on "Fonts" in the left-hand side menu
Click on the font that you want the minimum size to of the sign to be set for
Scroll down until you see "Minimum height of uppercase" and "Minimum height of lowercase" inputs. This is where you can set a value of both the uppercase and lowercase characters e.g. 6cm and 10cm respectively, now when that font is selected by a customer the sign heights minimum size will be that value.