In order to enable the FontSlider functionality on products, first you need to select or make the field, then open Field Settings and follow this sequence:

1. Click on Properties.
2. AllowFontSizeChange should be checked.
3. MaxFont: Maximum size that customers can choose.
4. MinFont: Minimum size that customers can choose.

P.S:

1. Also keep in mind that this function can only be applied to "Fields" not "variables" and "Fit to Box" should not be checked.
2. If you can't see these options please make sure you have downloaded the latest version of "Infigo Designer"
    https://infigosoftware.zendesk.com/entries/22523157-Infigo-Designer-Installers-Upgrades