If you have a large .miapp file that you are trying to upload to the NextGen Designer you may see a message similar to the one below:

This indicates that your server's web services have disallowed the upload size. On a new installation this defaults to about 30MB. To up this limit, you will need to change a settings on your server's deployment.

First, within your Internet Information Services (IIS) Manager, locate "Request Filtering":

Open that feature and click "Edit Feature Settings" on the right:

Change the "Maximum allowed content length" setting from its default of 30MB to something larger such as 100MB. Note that this setting is specified in bytes, so 100MB would be 100000000.