Backgrounds, sizes, resolutions, page load speed

I am in the process of making some custom backgrounds for Wordpress.

So I was wondering, if there is any convention for full-screen backgrounds.

Questions:

What size should ‘full screen’ backgrounds be?

Does it matter if I design them as vectors or rasters in terms of low-filesize output possibilities?

What sort of filesize is regarded as ‘good’ for a full-screen background? (I am talking about the top and sides of the display - not the text area in the center of the screen. In this regard “full screen” means as large as the display.

Any tips or tricks on how to achieve geometrically large images that have relatively small filesizes?

The ideal is to use an image that has a repeating pattern/gradient etc, so that you can make a small slice of it and let it repeat across the screen. The last thing you want is some gargantuan background decoration slowing down the whole site. (Don’t forget mobile users, who pay a mint for bandwidth.)

If you are forced to use one bit image, I’d be trying to get it below 100kb. There’s no point chasing a mega-wide screen, either. Rather, choose a nice background color that the image can merge with if the viewport is wider than the image.