First of all, let's take a look at a personal FB page with the new timeline setup.
Here's my personal FB page. Do you see how my cover photo doesn't start at my personal photo? It begins behind the personal photo, about 2/3 of the way down.
Your FB business page will also begin the same way. So if you try uploading your regular blog page header, it will get cut off behind your page's logo or photo (in my case, it's a picture of me).
Now, to make things easy, I wanted to simply use my blog's header. My page size, in Blogger, is set at 1040 pixels. I knew I didn't want my header to be the full length of my blog's size, so I settled with 875 pixels wide. The height of the header can be any size. I chose 200 pixels.
To ensure that my perfectly sized header wasn't stretched out at all by Facebook's page editor, I used a blank canvas that was wider than my header. The size I used for my canvas is 926 x 442 pixels. I had to play around with the size of the height and width until I got the size that I wanted and that looked good on my page.
If you want the blank white canvas I used to create my cover photo, click here. Right click on the white rectangle and select "save image as". The file is a JPG image, so you can open it with Photoshop or whatever program you use to create your header and other blogging images. I use Microsoft Digital Image Suite 2006 (which is no longer available for purchase) to create all of my blogging images. I'm going to attempt to learn Photoshop this summer though.
To upload your new cover photo, hover the area where it belongs and a "change cover" button will appear in the bottom right corner of the section. Select "upload" and wait for it to load. If the image is bigger than the cover area, you can reposition it to show what you want.
If you have any questions, let me know!