Although you can use any size for a background image, if your image is too narrow you may get a tiling effect. The height of the image is less of an issue as your page will expand based on how much content you add to it.
Try an image of 1500-1700 pixels in width to avoid tiling. Please keep in mind though the larger the image size, the longer it will take for your image and page to upload. Please be sure to optimize your image.
You can use the following Image Compression Tool to optimize your image. Simply upload your image, compress and download the smaller compressed image. Then you can upload this smaller image to your file manager and add it your site.
If you have access to image editing software such as Photoshop, you can use their "Save for web" function and this will automatically optimize your images and save them in a web-friendly format.
You may be interested in these background image generators:
- The Tartan background pattern generator: tartanmaker.com
- Stripe generator: stripegenerator.com
- Ribbon generator: quickribbon.com
- Palette generator (generates a palette based on a pic) if you want to match your background to a color on a pic: bighugelabs.com
- Gradient Image maker: tools.dynamicdrive.com/gradient
- BG Patterns: bgpatterns.com
- Eos Development: eosdev.com
- Hex color generator: cloford.com
- Pattern Cooler: patterncooler.com