Hi Brisotti,
For that specific badge, the selector is .h-member-badge-350043526109298115
so the font can be controlled with something like this:
.h-member-badge-350043526109298115 { font-size: 0.8rem; }
Note that we don't generally offer free assistance with Custom CSS. Custom CSS assistance falls under our Advanced Support, which is $150/hour. I've made an exception here to get you going in the right direction. If you need further assistance, please open a new support topic.
Regards,
Brian