Background image overview
Background Repeat: This manages whether or not you want your background image to repeat or to "tile." The first option is no-repeat which means that it will only show your image once. Repeat-Horizontallymeans that the image will repeat left to right only. Repeat Vertically means that the image will repeat up and down only. Repeat Horizontally and Vertically will tile your image and repeat unlimitedly in all directions.
Background Attachment: This is a new feature. It allows you to fix the background image to viewing window. Fixed To Window will allow you to scroll down the website while the background image stays in place. You wouldn't want this option if you are using a background image that repeats vertically. Scroll with Page will scroll the background image with the page as you scroll.
Backround Position: This options lets you determine the starting position of the background image. Most people choose Top Center. But you can choose whatever you like. If your background image repeats, this position will specify where the very first instance of your repeat starts.
For the size and dimensions of background images, I personally like to make the background width about 1700 pixels wide by 800 pixels tall. This makes the image look good on wide screen monitors. This would be way too big for images that repeat. Repeating images can be a variety of sizes depending on the patterns. For an easy google search on repeating backgound images, search "background images" or "background patterns" in images.google.com.