Add max size / scaling restrictions

[dcherman]

Hi,

It looks like there is no way to restrict the max height/width or scale of incoming requests, so users can send requests that can either exceed max int, or they can potentially send requests large enough to cause servers to go OOM attempting to handle them.

We should add support for maximums for each of those values with the default values being the ones documented on the export server's demo/landing page.