Will 15" SSD size increase above 768?

Highly doubt it.
Considering 1TB SSD/Flash is still extremely expensive (over 1000USD), it wont for a while.
My guess is in next 2~3 years it will be available.

 

What about the Crucial M500?

You very well may be right, but I don't think it'll be that long. Seems to be moving quicker considering a 600$ 1tb exists. Maybe 1 year away I'd say

 

I am pretty sure Apple still uses the 64bGbit chips. If they used the 128Gbit ones that are in the Samsung 840 and exist also in non TLC version already they could double the capacity by just switching out chips.
I think a 1TB option is almost a certainty.

1TB are really expensive because the highest capacity SSDs always have been proportionately ridiculously expensive. With 128Gbit chips out in force I don't see anything stopping them from going for 1TB. They will probably not go for 1.5TB (2x 768) but rather use the space savings.

 

Since rMBP uses a flash chip it might be a bit expensive.

 

$$$$$$$$$$$

 

It would probably be too small a seller for the moment, but it would be nice to see capacities increasing...

 

Apple is using proprietary PCIe SSD's in the Airs and presumably would do so in the MBP's. I wouldn't be surprised to see a 1TB SSD as a BTO option in the next MBP update, especially if it isn't until October.