The size of Youtube one channel background is the following:
2120 x 1192 px
Desktop Full Zone:
2120 x 350 px
1536 x 350 px
Desktop Small Zone:
1280 x 350 px
Each zone represents the area that will be visible in the specific devices. i.e. TV zone is 2120 x 1192 pixels.
You can download a free template with the correct size for youtube one channel here.