You can use "Overlapping at offset" event.
If your character is crouching, on every tick check if its "Is Overlapping Ceiling at offset (0,-10)" for example.
If overlapping, remain crouching, else set full size.
Or probably an easier solution would be this:
On every tick
Character set size (full)
Character Is overlapping Ceiling -> Character set size (crouching)
Because it's done in one tick, the character will only be rendered once - either standing or crouching.