At MBB, very unlikely you'd have more than a high level, top down, back of the envelope market sizing exercise; even that is fairly uncommon in my experience.
What's the optimum way to prepare? Get a coach. Short of that... focus on the case mechanics, which are portable skills you can reuse no matter what case you are being given. I'd focus on understanding a specific type of case only much later in your prep, if ever - you have to be ready for the unexpected. If you know how to think, you'll be fine.
PS: Expect to spend over 100 hours on your prep; case interviewing isn't hard, but you need to make it second nature so you are able to really focus on cracking the case during the interview, vs. struggling to remember what you are supposed to do when.