Background for website

I was trying to change my template's background last weekend, but was stumped. I already have an idea what I want but putting it on the template and getting it to work stopped me. It was either too big, or too small. I wonder if there's a fixed size for it....?

Anyway, it looks like I might need to do some more research on how I might add my own background to my website. Will definitely do that, when I get the time. lol. In the meantime, I think I need this book... LOL