When you enable SSL on IIS versions 6.0 and newer, there is a limitation in the file size that a user can upload. Microsoft introduced this feature to improve security and prevent certain types of attacks on the server, however this can present a problem when uploading a form template (MFT) that is larger than the default 29kb.
You can modify 'Request Filtering' to allow larger file sizes. See the Microsoft provided page on Request Limits to see how this is done in several different OS's and Pay particular attention to the maxAllowedContentLength property.
Also the following image shows where this property resides on my local IIS server:
Once you change the maxAllowedContentLength property to something more appropriate for your needs will allow you to upload larger MFT files.