Just purchased and installed and get blank screen when clicking "Settings"

If you have not already done so, you can go in and delete the gravityforms plugin folder via FTP: the should get your admin area back. Once you have done that, you need to fix the problem which caused the blank screen. Sounds like you are running out of memory. We need to know how much memory is allocated to WordPress and PHP, or you can just ask your host, or configure it yourself, to get 128M of RAM (preferably.) Here is one explanation of how to do that http://www.nerdgrind.com/increase-wordpress-and-php-memory-limit/

If RAM is not the issue, you might have a theme or plugin conflict. If you have sufficient RAM, but still cannot access the Gravity Forms settings, you will need to check for a theme or plugin conflict by following these instructions http://rkt.gs/testing

It's normally one of those things: PHP memory, plugin conflict or theme conflict. It could also be due to a bad installation, in which case reinstalling is the answer. That is far less common than the other problems I described.

There is no limit in Gravity Forms. You are limited on the server based on the max execution time of the script (a PHP setting) and the RAM (a server setting.) There are other specific restrictions as well, like maximum POST length, but those restrictions are not part of Gravity Forms. They're all on the server. You can also run into an issue in your browser when manipulating large forms, based on the amount of free memory and processing power on your desktop. There is a lot of JavaScript on the client side, in the browser, when building and editing a form. Again, it's not a restriction in Gravity Forms at all.