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Is there a way to change the outgoing mail font size?
You can change the size as it appears to you, but I don't think it changes how it appears to the recipient.
Go to settings > Display and Brightness > text size. There's also a setting in Accessibility. Both do the same thing, but the one in Display and Brightness is in smaller increments. I'm not on iOS 11, so it may be different from what I have on iOS 10.1.1.