Norwegian company Lufttransport have a scheduled route BOO – VRY using the AgustaWestland AW139 helicopter. This helicopter has 15 seats. You specified smallest, so here are the dimensions from Wikipedia:
Length: 16.66 m (54 ft 8 in)
Width: 2.26 m (10 ft 0 in)
Height: 4.98 m (16 ft 4 in)
Empty weight: 3,622 kg (7,985 lb)
For comparison, the Twin Otter DHC-6 Series 100 has:
Length: 15.77 m
Width: 19.8 m (wingspan)
Height: 5.9 m
Empty weight: 2,653 kg
So although the AgustaWestland is a bit longer and heavier than the Twin Otter, it is less high and certainly much less wide. So whichever is smaller depends on how you want to define the size.