All my backgrounds are made in size 1600x1280 px to fit most screens. They work best with the Minima Template, but also with the new templates in blogger.
I only use material I have gotten permission to do so with. So far I have only made backgrounds for a 2 column template, but if you find something you like and want it a 3 column I might be persuaded to make one for you. Do not hesitate to drop me a line, either in the comment section or send me an email.
All my backgrounds are for personal use only. Please respect the integrity of the designers and me.