At this time I'm not aware of any manufacturers that make larger chips. Take a look at the teardown here and you will see that the Retna based MBP no longer uses a user replaceable drive/RAM.

no, it is flash storage and is not able to be removed from all i heard..... id go with the 15" none retina, it is able to be upgraded at our will and u can remove the cd drive (if you have skills) to chuck in a secondary harddrive.

To answer your question about form factor, no, the Retina MacBook Pro does not use a standard 2.5" profile. It is a proprietary Apple design that will probably take 3rd parties a while to copy.

This is unrelated to your question, but why do you want/need so much flash storage? The maximum configurable 512 GB should be good for anything you need on the go, and you can always use a 1 TB or larger external HDD at home.