You can NOT with the stock GMAIL App ... but you can use other 3rd party APPS that provide that ability to access you GMAIL account --- K9 email is one that is popular, there are others as well ... just search in the APP Market for email Apps and read the various features that you want.
I do have the Droid Razr. I was able to change the text size for GMAIL.
Do me a favor, go to the GMAIL account, select Menu,Select More, Then go to Settings, under Account Settings, you will see your GMAIL email account, go down to Message Text Size. The largest option is Huge.
It doesn't give a "font size" however, you will notice that the text in the emails is bigger. Now I don't know if it's big enough for you, however if the text isn't large enough, then you can do what Cody21 has recommended earlier (download an app that will change the font size)
The exact same steps apply for the Droid X.
I'm sure you should be able to find a font size that is suitable for your needs.