to determine your current screen size:

1. go to Start / Settings / Control Panel / Display and click on the Settings tab, OR right click on the desktop, select Properties, and click on the Settings tab
2. your current screen size is at lower right

to install new wallpaper:

1. click on one of the screen sizes (like '1024 by 768' or '800 by 600') to open the full-size image
2. right click on the full-size image and select "Set as Wallpaper"