Hello Omar,
It depends on what result you are trying to achieve: to make sure you can see the text on your small device, or to make sure that the recipient(s) of your messages are forced upon with the
LARGE FONT, even if they might be reading on a laptop or desktop.
Setting up large font size for your message will achieve the latter. Luckily, the sender has the power only over the message body, and not over the headers of the messages. And fortunately, Kostya (Aquamail sole author) has limited the font size to 14pts to spare poor folks on the other end. (And personally, - as much as I can, I limit my e-mail communication with those who set the fonts to any preset type and size, because the sender knows only how it looks on his/her screen, not on mine.)
If it is the former, - please read the FAQ for Aquamail:
http://www.aqua-mail.com/?page_id=227 Among other answered questions, it has "When viewing or composing email, the font is too small".
In addition to the recipe offered there, you can open the "3-dot" menu -> View -> and then change the font size.
Hope this helps.
