1. use magic wand tool to select whole image...
2. Use the "move selected pixles" tool, (top right of tool box) and you will see 8 dots appear around your image
3. Use these "dots" to drag the image to a certian size...
ORRRRR
Open your image in paint.net, and resize the image to what ever you want, deselect "Maintain Aspect Ratio" to have full control over what size you want it...
Hope i helped!