The Droid 2 keyboard and Android OS 2.2 "Froyo" is very nice, but still help in the fonts department. And before you iPhone folks chime in, the fonts on the iPhone SMS look about the same, though I'm not sure if they can be changed.
Anyway, you can't tweak the default SMS application. And the "magnifier" on Adroid is silly / useless imho.
However, there is a REPLACEMENT SMS client for Droid that DOES let you adjust fonts, and it's FREE!
ChompSMS / Chomp SMS
Long story, but no rooting required. It talks about "font packs", but you probably have some on your phone already. Even the System Font can be increased in size, and also you can have it render as Bold.You have to let it search for all the fonts in your phone at least once; it will find the built in System fonts, among others.
You can also adjust the text color and background color.
Oh, it can also be set to REPEAT the text message alert, including a few different vibrate patterns. And you can put it's icon on your desktop screen.
I just wanted to get the word out for folks with poor vision, or heck, who aren't 20 years old with aparently 20/10 vision.