You can make the font size bigger with no problem, but making it bold would require some tinkering with the text configuration files.
Just go to the Mail and News settings and then Composition and you can set the font size to something larger. If you want an option for making it bold, submit a feature request via Bugzilla. It's probably a pretty simple matter to add, but very few people want it.
Want to make my typing in emails to be BOLD and LARGER than the default type setting. Have looked everywhere for these settings to change but cannot find them. It is cumbersome to have to change settings for each email and then they often do not hold from line to line in a paragraph.
Am running XP Pro but the XP forum was unable to help me and I seem to have lost the post there. My email is Mozilla and I use OE browser.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic