Sorry, but your link requires a login, so I can’t see it.
First, restart your browser.
If that doesn’t fix the issue, my first guess would be a plugin or theme interaction, since the size of that window is controlled by a commonly-used jquery library. If you can do it, the simplest way to id that would be to disable all your other plugins, switch to a basic wp theme, and see if that fixes the issue. If it does, re-enable one at a time until you find the culprit.
If you find such an interaction, please let me know so I can see if there is any way to get around it.
If that does not work, I’d need some more information:
1. Does the same effect occur in all browsers, or just one?
2. Does it occur for all error messages ( try omitting a required field ) or just this one?
3. Does it occur for all webforms, or just one?
4. Do you see any javascript error messages? ( in Chrome, right-click in the window, select “Inspect” and go to the “Console” tab )