I'd go with your intuition. Stand close to her and have her extend her arm and flex her muscles. Then show her how much larger yours are and say, "I'm not trying to compare but the laws of physics just can't be denied. Size matters and I don't want to prove it." You could show her you fighting another man your size, and when the fight is over, ask if she wants to fight that man. Of course she won't, so then your point is proven. If she wouldn't fight him, then she should not fight you. Unless she expects you to treat her with kid gloves and just help her practice.
It's more important to preserve the psychological dynamic between partners. Think of the problem as a car leaking antifreeze while parked. It would be horrible if a cat licked the puddle and died. But it would be even more horrible if it was your roommate's cat in its own driveway licking your puddle and dying. Better to park down the street. (And put a bucket with a large rock in it under the leak to deter animals). That way if anything happens, at least it won't ruin your household goodwill.
I would "park down the street" whenever she wants to spar.
