Norwegian company [Lufttransport](https://en.wikipedia.org/wiki/Lufttransport) have a scheduled route [BOO](https://en.wikipedia.org/wiki/Bod%C3%B8_Airport) – [VRY](https://en.wikipedia.org/wiki/Værøy_Heliport) using the [AgustaWestland AW139](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/De_Havilland_Canada_DHC-6_Twin_Otter#Specifications) 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.