When setting your blog, many of you have someone like Michelle Laycock create your banners, however, many have created their own and asked "what size should my banner be?"

In general, many banners have a max width of 900 pixels (px).  And the height could vary some from 200-250 px depending on if you have navigation buttons/bar added to your banner and so on.
 
For typepad users, 870 (pixels) fits perfectly in the middle if you use a 3 column format, 200 each sidebar and 500 for the main column. 
 
For Blogger, the width depends depending on if you customize the html code and such. but it's always best to start with 900 and then resize it smaller if necessary — that way you don't loose image quality.  If you start too small and resize it bigger…the image quality is reduced/poor and becomes what is called "pixelated" (where you can see the squares, pixels, that make up the image).  If you have a 2 column blogger format, then 700 px is a good width to start with.

For Constant Contact, create a banner with a width of 600 px.
 
Again, this is just general information about banner sizes and may not apply to everyone — but I wanted to share since the question had been asked.

Happy blogging!