OK it needs a character, powerups and enemies.

But how many rooms, powerups/keys would be just enough to provide a touch of adventure?

Fundamentally a metroidvania is characterized by nonlinear levels and utility-gated progression.

For example unlocking double-jump should allow the character to progress in an area that was previously accessible to the player but not traversible.

A dash can be unlocked to avoid enemy attacks but also blast through weak walls.

My point is don't focus on getting keys that unlock paths (though you can definitely fit that into your game to some extent)

Have a look at this page https://en.m.wikipedia.org/wiki/Metroidvania

As far as length goes I believe for the jam you should aim for 20 minutes to 1 hour of gameplay. For example I don't have much time to play so I doubt I'll spend more than 15-20 minutes on each game. It's better to have a shorter more polished entry than a game that takes 5 hours to complete but it's repetitive, un-inspiring or even buggy.