I would say create the largest possible (within reason) and then offer various resolutions for each file. The ones graphic mind pointed out a pretty standard.
I would say its a personal preference thing, Obviously you don’t want to be doing them in a small resolution like 800x600 if you’re going to be selling them on the marketplace but the above suggestions are good. I would work as big as you can but not overly big Just enough so that you have the option to downsize to the standard resolution sizes if you wish to sell them like that or have the choice of selling the larger copy.
Perhaps it might be an idea to take a look at 10 - 20 of the top sellers in the backgrounds category to get an idea of what sizes they are using and including and then from there you can decide what resolution you want to use for the end product for sale.
For reference here are common desktop resolution sizes -