Hi Chaitanya,
There is no recommended background size, as there’s really no right — or at least no universal — answer to what background size you should use for every project.
Once you’ve uploaded and selected an image to use as your background, you’ll be able to use different presets to display that image, like Fill Screen, Fit to Screen, and Repeat. The size of your image is up to you, but it’s probably best to consider which preset you want to use. For instance, it would make sense to choose a larger image if you’re using the ‘Fill Screen’ preset than if you’re using ‘Repeat.’
If you’re looking to use a preset like ‘Fill Screen,’ what you’ll want to do is find an image that is large enough to look good at different resolutions, without being too large to significantly hurt your site’s loading times. If you’re using an image that is 500 pixels wide, this might quickly look distorted at larger screens, but one that’s 1400 pixels wide for instance might look much better.
I can’t give you an exact size that will work best, though, since this doesn’t really have anything to do with what theme you’re using. It really depends on the screen resolution (and width of the browser window) that you and your users are visiting your site on. If you want some more ideas or tips, you might Google some statistics on common resolutions for web design, or look for more info on how to set up your own analytics to see how people are viewing your site to get personalized stats.
