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.
This free Support Forum relies on contributions from our support staffs and fellow Graphene themes users. While we endeavour to provide adequate and timely support, due to its voluntary nature responses are not guaranteed and may be delayed.
If you require urgent assistance, you may like to consider our Priority Support option.