photoshop help

You were probably looking for actual dimensions but unfortunately it isn't that cut and dry. I completely agree with James but would like to add a couple points.

Personally I've really gotten attached to the flat design look and prefer solid colors but on occasion I'll use actual image backgrounds. When I do, I like to use small tiling textures (tons can be found here http://subtlepatterns.com/).

If you don't like the tiled look you can try using the cicada principle (I can't remember but I might have learned this on treehouse - (http://designfestival.com/the-cicada-principle-and-why-it-matters-to-web-designers/)).

Keep in mind that people will most likely be visiting your site from a mobile device so loading large images won't be a great idea. Using James' suggestion of responsive design will allow you to set media queries where you can swap out a background image for a solid color.