Background no repeat

I have verry big problems to fit a background is there some orginal sizes?
I wanna make a background whit out repeat. Im having big problems to make it fit where can i read about this or is is some kind of guide for this?
I have a picture that i like alot and its in 2560x1600 png but it dosent matter what i do i just dont fits the screen it gets super small i have been sitting with this alot and dont find anything about what i can do.