The 5Mb size limit is not going to be increased. This is the limit for the size of the aia file. The limit is in place because of a Google imposed time limit (60 seconds) on the use of their Cloud server (which MIT uses). MIT has no control over the terms of us and restrictions on that server.
It is possible to build apps off line using any of several third party App Inventor clones... of the order up to about 10Mb. Do so at your own peril. Doing so and the app you build may not be compatible with MIT App Inventor online. The choice is yours.
Regards,
Steve