@danniee,

Try this CSS code. Put it in Dashboard > Forums > Settings > Styles Tab > Custom CSS Code textarea, save it, delete all caches, and use private/incognito tab of mobile device browsers to avoid those hard caches and see the changes:

@media screen and (max-width: 1000px) {
    #wpforo #wpforo-wrap .wpforo-login-wrap .wpf-field .wpf-field-wrap input[type="text"], #wpforo #wpforo-wrap .wpforo-login-wrap .wpf-field .wpf-field-wrap input[type="password"], #wpforo #wpforo-wrap .wpforo-login-wrap .wpf-field .wpf-field-wrap input[type="email"], #wpforo #wpforo-wrap .wpforo-login-wrap .wpf-field .wpf-field-wrap input[type="checkbox"], #wpforo #wpforo-wrap .wpforo-login-wrap .wpf-field .wpf-field-wrap input[type="radio"] {
        font-size: 17px!important;
    }
}