How to resize image to specific length

Hello,

I would like to re-size an image to a specific length (or height), while 
keeping the aspect ratio preserved.

How do I do that?

My naive attempt was to grab the left upper corner of the image (using 
control point snapping) and move it to the desired location. Then I'd 
grab the right upper corner with the resize tool, hold shift, and move 
it horizontally to the desired position (using grid snapping).

Unfortunately this does not work. While the cursor snaps to the current 
position, the corner of the image actually "drags behind", so it ends up 
larger than it should be.

Best,
-Nikolaus