To change the size of your wallpaper (also referred to as Desktop Background) in PCs running Microsoft XP you can either modify how the photo displays under "Desktop Display" or you can actually modify the photo size using photo editing software.
Change Desktop Display Options
Click the "Start" button on the bottom left side on your computer. Select the "Settings" option. Click on "Control Panel." Once the Control Panel window appears select the "Display" icon. The "Display Properties" window will appear. Select the tab that says "Desktop."
In "Desktop" you can select various options that include changing or uploading a new photo to appear as your desktop background. To change the size of the photo used as your desktop wallpaper, look for the header "Position" that appears on the middle right side of the window. Click on the downward pointing arrow to change the size of the photo from "Stretch," "Tile" or "Center."
Using a Photo Editing Program
Open the photo you'd like to set as your desktop background in a photo editing program (e.g., Paint, Photoshop, Quark). Depending on the photo editing software you have, look for the options "Photo Re-size" or "Change Image Size." Once you select this, you'll be prompted to insert new dimensions (either in inches or pixels) for your photo. Once the photo is re-sized, re-upload the photo under the "Desktop" tab under the "Display Properties" window and select the new image to set as your desktop background.