The font size of Rich Messages is defined in the style sheet attached to the specific rich message in the Punchh Platform. You can attach a style sheet by navigating to the Punchh Platform > Whitelabel > Style Sheet.
The CSS code within the style sheet will include the line "font_size": xx. The values entered for "xx" will set the font size. See example below:
If the style is not configured in the rich message, it picks the default font size hardcoded in the mobile app framework.
If you are not able to access the Whitelabel section of the Punchh Platform, please submit a support ticket to https://support.punchh.com