Thanks for the quick response, Plamen! Well, I know the Rows property, but I don't know in advance how long the text in the textbox will be (depends on user entry), so I don't know in advance how many rows the textbox will need.
The point is that I want the textbox to be EXACTLY as high as required when the page is in read-only mode (in edit mode the height doen't matter). So it seems that I have to count the characters in the textbox and set the number of rows dynamically.
Disadvantage of this approach: the calculated number of rows can only be an estimation, especially because Internet Explorer breaks the line after a hyphen, but Firefox doesn't.
So I'm still hoping that the radTextbox has some kind of built-in feature for that purpose...