A link would help if you were willing to read it. The information you are looking for is on Help:Theme_Designer. We are here to help you with things not covered by the help pages. We are not here to be the help page.
The Fandom background can be any size you want but the file must be less than 300 KB. If you want to have the background image cover the entire page without the vertical color break or tiling, the image must be at least 2000px wide. The combination of these two requirements usually means images need to be compressed. This can be done using image editing software and/or online services such as compresspng.com.