If you're talking about for email...
Switch to the email account whose settings you want to change.
Press MENU and then tap More > Settings.
Set the font size when reading emails, enable, disable or set the signature, set the account as the default email account, and set where to save email attachments.
There is not a general default text size for the phone. It is more of a per application setting. For example, to get larger text in the browser, you can open browser> press menu> press dots> text size> set how you want.
I hope that answers your text size inquiry.