There are already active items that make you fully immune to damage (not just contact damage). That said, they are time limited as opposed to being functional as long as you stay in the room. However, my thought was that still being vulnerable to projectiles would help balance it, especially since it would also increase your size 3x, which makes it harder to dodge projectiles due to the increased hitbox size. Iirc size up at least used to increase hitbox size, though I think it was capped to stop you from being soft locked because you were too big to navigate the rooms.