My Tumblr Backgrounds come in one size 1680x1050. But if you need it smaller you can download my Tumblr Resizer Action for Photoshop here.
To Download the Backgrounds just pick the background you want and click download. Or you can click on the image, Right click and Save As.
Preview opens at 1680x1050.
Make sure you change the following line in your Tumblr Custom HTML:
top left fixed repeat to this top left fixed no-repeat.
If you need help with adding a background click here.
As always, my Tumblr backgrounds are FREE to download and use as you wish just don't claim them as your own and we are all good. :)