Messages: Go into the messages app, select a specific message, tap the menu key, select "... More", select "Text Size", set your desired size. "Largest" isn't huge, but it's twice the size of the default. When you've set this once, all messages will open in the selected font size.
Gmail (as opposed to HTC Mail, which as mentioned previously has pinch-zoom): Go into Gmail, tap Menu, select "More" then "Settings", under "Account settings" click on your account ID, set "Message Text Size" to something bigger.
EDIT: Whoops, just noticed I've been replying to a resurrected long-dead question. Hope it helps someone anyway... LOL