I can't see an easy way to do this with a standard text box, I've tried CSS styling the id and can't find an ID to just target that textbox, it's possible to target all of the textboxes, but I'm sure that is not useful!
One other thing to try is:
Use the RichText editor
This should bring up the FCK Editor
Go to view your profile (logged in as admin)
Click on Show Custom Editor Option
Settings type at the top - select "instance"
Other editor options
HTML Encode Output select No
Reverse HTML select no
Force width and height, you can specify the dimensions you require
Editor toolbar view options - in there you can remove the toolbars to strip down the editor
At the bottom, select Instance and click on Apply - These settings will just be applied to the FCK Editor used for Profile Properties.
You may find it useful looking at these tutorials for further configuring the FCK Editor:
Let us know if this works, thanks,