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.
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NOTES
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');