So apparently there is one user (Xelna) that has a background in his/her profile that has an entire image.

Since I'm interested of putting my own image in my profile, does anyone know what is the size of the background?

Xelna isn't the only one that has that. The background gets resized with the size of your screen (admin should correct me on this if this is wrong). So if you like a nice non pixelated background i would recommend a 1920¤1080 size.

So if I put a 1920x1090 image on my profile, would the background be resized later on? I assume it doesn't do it automatically.