In researching this, it appears that RadRotator does not directly implement a resize method within its client-side API.
You can use the RadWindow Client-Side API
"set_height" and "set_width" in pixels to an appropriate size for each new image.
Then you can use the window.onresize event to explicitly set the "style.width" and "style.height" properties of the RadRotator element based on the dimensions of each new image pulled from the database.
You can reference the RadRotator resizing issue
forum thread for insights on doing this. And, also refer to the help stretching the rotator horizontaly
for a better understanding of resizing the RadRotator.
Hope this helps you.