Hello Richard,
Setting full screen backgrounds can be a bit tricky sometimes. Depending on what the content of the background is, you may have to think beyond just the suggested dimensions.
If the illustrated portion of the background is too large, what I would suggest doing is adding a bit more white space to the perimeter of the image. That will force the illustration to be smaller and not so large when it’s displayed.
The background is always going to fill the entire screen. So if you need an element in the image to be smaller, you have to make it smaller within the image itself.
The image will also always be set to the vertical and horizontal center. So any portion of the image that is cut off will be on the left/right or the top/bottom. As you lay the image out, work from the middle of the image instead of the top. That way the most important elements will always be visible in the middle.
Try giving that a shot and let me know if you have any questions,
Bill