That’ll be 2560×2048 according to Wikipedia, but it’s impractical to set a background with that resolution, because the file size will be huge and will take forever to load (in today’s relative term at least).
There are a lot of monitor sizes out there with a lot of variation in their resolution. I would recommend a dimension of 1920×1080, with a JPEG quality setting of maybe 60 or 50. That’ll cover most monitors and resolutions, while keeping the file size reasonable enough for high speed connection.