How to make your header fit.
There are a couple possibilities. Create a new header in a photo editing software and create it exactly the width you want it. Then in blogger Layout click edit the header and upload it with the option: INSTEAD of title and description.
Then go back into the HTML layout code and find:
And delete the line that includes: border: 1px solid....
Now there will be no line around the header either..
If you have a header but it's too short or too long, and you don't have a way to edit it... you can upload your header into Picnik.com and use the resize feature. Be careful though, only a few pixels at a time or you will loose too much resolution when enlarging. Try it in 10 pixel increments.
Save to your computer and upload in the header editor as above.
You may need to change your Header-Wrapper size as well.