If you need to increase the font sizes when VIEWING gmail inbox/mails/quick reply box this CSS overrides the defaults, and i've commented the CSS to make it clear which elements control which parts of the gmail interface.
note, the font specified in the CSS is Lato which is a google font, but this CSS solution assumes you have the Lato font on your own system - rather than loading it from google. You could change Lato to another font that you do have, or perhap add an @import line to this CSS (not tried)
e.g. @import url('https://fonts.googleapis.com/css?family=Open+Sans');