In most cases like this, one of the first things you should do is check out the f7 css variables (or just inspect the element using your browser dev tools to find out if the property you want to change is set by a css variable).
In this case you see that there’s an --f7-button-font-size variable. So, the easiest solution to the problem is to use the style property in the button itself to change the value of that variable for the button and it’s label.