What is the biggest disk that can be used in place of the optical drive in an early 2011 15 inch MacBook Pro?
I added an SSD to my Macbook back in 2012, and did a write-up about it here: http://blog.omgmog.net/post/upgrading-my-macbook-pro/

Most 2.5" drives should do, the dimensions of these drives is pretty much standardised.

If you're putting a HDD fitted using an adapter in the old optical drive bay, it won't have the shock detection that the lone HDD bay has. Choosing SSD in the optical bay is better from a shock management aspect.