The best way to do this is to go into your email and change it manually. The steps to perform this are listed below:
-Press Menu button
-Select Message Text Size
-Then select the size you would like
This should enable you to change the font to whatever size you would like.
Unfortunately, that doesn't work if you are using the gmail app.
As for text messages, you may want to download Handcent from the App Market. It is free and allows you to customize the look and font size of your messaging.
First, make sure you downloaded the right Handcent app. You want the one that is Handcent SMS. If you did that, I don't know why it would tell you not to launch it. If there appears to be a problem, uninstall it by going to your task manager, touch Packages along top row, and you should see the handcent you have with an uninstall button next to it. Uninstall it and then go back into the Market and re-install it (making sure it is Handcent SMS)