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My theme is Oxygen and the basic header size is 910px for 150px, I want to make it larger, can anyone help me please???
The Oxygen theme supports flexible header sizes, so you should be able to upload a taller image and have it just work.
If you're trying to make something wider, then you'll need a bit more custom CSS for that because what you'll want to do is change the entire width of the main content container for the theme. In some themes, changing the width like that is pretty easy, but in others it takes some work. In Oxygen, it's quite easy to change the max width of the blog, however, it's much more difficult to change it and also have it work well if you have setup the showcase template like the http://oxygendemo.wordpress.com/ site.
Here is a very basic example to illustrate:
If you wanted to change the main content container width and also adjust for the showcase template and featured images, it will take more work but the example above should get you started for now if changing the entire container width was indeed what you were trying to do.