I Am Using Blogger's Template Designer
To adjust your blog's width to make it skinnier or wider;
Template > Customize > Adjust Widths > Entire Blog Width
Play with the width until it looks right. (It will be about 660px for a 2-Column and about 940px for 3-Column)
I Am Using an Older Version of Blogger and I have a 2-Column Layout...
Any 2-Column or 3-Column layout found on Dotty Dot Dot FREE Blogger Backgrounds will fit.. 2-Column Backgrounds will have a tighter fit on a 2-Column layout. 3-Column Backgrounds are extra wide, but they still look good on a 2-Column layout.
I Am Using an Older Version of Blogger and I have a 3-Column Layout...
Any of the wider 3-Column Backgrounds should fit. BUT 2-Column backgrounds are too small for the wider 3-Column layouts. *If you don't have a 3-Column layout, but you want one, use these instructions.
Read about using different sized HEADERS on different sized layouts, here.