The maximum screenshot size depends on your package. However for the Professional Package and above you can create a screenshot with a maximum width of 10,000 pixels and an infinite height, but remember the browser width and height needs to be bigger than the screenshot dimensions.
While a screenshot height is stated to be infinite in reality there is a maximum height of 65500 pixels due to operating system restraints.
If you use the full length and full size screenshot options then the height of the screenshot can be anything up to the hard limit of 65500 pixels. As the screenshot matches the height of the web page that is being targeted.