Screen sizes can vary widely, you should download the image for whichever ratio you are using and your computer should reduce it automatically to fill your screen. Just check the guidelines below against the resolution checker tool so you know which to download:

5:4 (SXGA) 1280 x 1024 (Common on 17" and 19" Notebook LCDs)

4:3 (Standard) 800 x 600 | 1024 x 768 | 1400 x 1050 | 1600 x 1200

16:10 (Wide Aspect) 1280 x 800 | 1440 x 900 | 1680 x 1050 | 1920 x 1200

16:9 (HDTV) 1280 x 720 | 1920 x 1080 | 2560 x 1440