Which character has the most to lose? Which character's choice(s) are the ones that drive the story into the next scene, making the reader want to turn the page? Which character's goal/motivation/conflict complements or opposes the previous POVC's in the most interesting/engaging/story-driving way? Which character(s)'s internals benefit from being left unsaid, and only filtered through other characters' eyes?

That's what I find choosing the right POV is about. On scene-level, chapter-level, book-level, whathaveyou; it's about whose story it is and why, and who'll have the strongest impact on the story, arcs, and the reader alike.

As for lengths of scenes, I think that completely depends on the story and what it needs. You probably want some sense of consistency, but if you try to always force your scenes into a specific (and same) length you're running the risk of messing up the pacing. Focus on scene structure and character arcs within, rather than numbers, regardless if you're writing mPOV or not.