You can set the font size, color, etc. of the Submit button in the Themes tab. 

Alternately, you can use CSS such as:

.action-btn.Submit {
    font-size: 18px;
}

To make sure the form looks good for mobile devices, you can simply check the "Optimize Forms layout for mobile devices checkbox". To specify the way your Form appears to different resolutions, you can use @media queries, such as the below, which specifies styles when the screen width is between 520 and 699 pixels.

@media all and (max-width: 699px) and (min-width: 520px) {
 .action-btn.Submit { 
   font-size: 18px; 
 }
}