As you noted, one case is going to be a market sizing, and one business case.
In terms of market sizing, the question is likely to be more of a "top down" market sizing (e.g. overall market size for hamburgers in the US) vs. a more "micro / bottom up" market sizing (e.g. revenue for a restaurant). The latter is more commonly asked at Bain interviews.
In terms of case - overall the case is likely to be the "traditional" candidate led case, but note that the line between the two has been fading and you shouldn't be at all surprised if the interviewer guides you in a certain direction / has questions in the middle of the interview.
Finally, the process of first round usually (at least until a couple years ago) also involves a "GMAT-like" test - the questions are actually very similar to the GMAT (more so than the BCG test, for example) so looking at some GMAT prep materials is the best way to prep.