I did a search of all intra-European flights for tomorrow (12 Feb 2018) and came up with the following:
Hahn Air has a flight with a Cessna Citation (6 seats) scheduled between LUX and DUS Loganair
has several flights, as previously noted, mostly from KOI, NRL, and PPW on Britain-Norman Islanders (8 seats), as well as a number of flights around Scotland on 19-seat Twin Otters Direct Flight has some Islanders with 9 seats flying out of LWK Hebridean Air Services flies some Islanders from COL, TRE, and OBN Isle of Scilly Skybus flies 16-seat Twin Otters from ISC to NQY and LEQ BA's Scandinavian subsidiary, Sun Air of Scandinavia, flies 18-seat Avro J-31's between BGO and BLL Aurigny
flies
18-seat Dornier 228s between GCI and DNR Flybe flies 19-seat Avro J-41's between CWL and VLY Chalair flies Beechcraft Beech 1900D's with 19 seats around western France, mostly based out of BOD and NTE Norlandair flies 19-seat Twin Otters in Iceland AIS Airlines flies 19-seat Avro J-32's around Sweden, including ORB, GOT, and MMX Aero VIP
flies Avro J-32's around the Madeira Islands Twin Jet
flies Beechcraft Beech 1900D's with 19 seats around France based out of ORY, MRS, TLS, and LYS SUE Orenberg
flies
20-seat Let 410's around Russia.
That's just for one day.
It's certainly likely that a lot of small routes with small planes don't fly everyday
so I'm probably missing some.
travel.stackexchange.com/users/66333/cbw
OFD in Germany is offering regular flights (scheduled) between Emden and Borkum using a Cessna 172, with 2 pax max.
Flight schedule
The fleet
travel.stackexchange.com/users/61784/Tony Delaney
Wikipedia lists a few dozen European operators of Britten-Norman Islanders .
Many of them are historical, but a few appear to currently fly scheduled routes with them: Loganair for Westray to Papa Westray Isles of Scilly Skybus Hebridean Air Services FLN Frisia Luftverkehr
travel.stackexchange.com/users/16947/hmakholm left over Monica
Norwegian company Lufttransport have a scheduled route between Bodø Airport and Værøy Heliport using the AgustaWestland AW139 helicopter ( Værøy Airport has been closed since a Twin Otter fatally crashed).
This helicopter has 15 seats.
It's a bit heavier and longer than some small fixed-wing aircrafts, but certainly much narrower.
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 The Britten-Norman BN-2A Islander is shorter, lower, and lighter yet, but as a fixed-wing aircraft, remains much wider than the AgustaWestland.
For most definitions of "smaller", the Britten-Norman would win, but if you're looking for the most compact "plane", the AgustaWestland helicopter flying to Værøy wins.
travel.stackexchange.com/users/2509/gerrit
In Portugal there is a company, Sevenair that only operates a small airplane, a 19 seats Dornier 228 .
It flies across all Portugal, North to South, connecting 5 cities, Bragança, Vila Real, Viseu, Cascais, Portimão.
travel.stackexchange.com/users/73683/Rui Barradas
According to their website , Finist'air has flights most days of the week between Brest and Ushant on a 9-seat Cessna Caravan .
travel.stackexchange.com/users/73645/David Moews
I’ve enjoyed flying in smaller planes on scheduled flights in the past (e.g. Aurigny’s old Trislanders). Are there any other scheduled flights in Europe in small (sub-20-seat) planes that are still running? I know about the Westray to Papa Westray flight already but feel free to include it in a list.
What are the smallest planes running scheduled flights in Europe?
travel.stackexchange.com
<air-travel><europe>
travel.stackexchange.com/questions/109647
travel.stackexchange.com/users/13172/Robin Whittleton
There are a number of flights in different countries with fewer than 20 seats, especially to remote areas and islands. These include a 6-seat Hahn Air flight between LUX and DUS; several 8- and 9-seat flights in Scotland using Britten-Norman Islanders, scheduled flights between Emden and Borkum in Germany using a Cessna 172, with max. 2 passengers, or Finist'air flights most days of the week between Brest and Ushant on a  9-seat Cessna Caravan. There are a large number of flights around Europe that are made in air planes of 20 seats or less with the smallest number of seats being 6. These flights take place in a range of countries including England, Germany, West of France, Scotland to name a few.
There are a large number of flights around Europe that are made in air planes of 20 seats or less with the smallest number of seats being 6. These flights take place in a range of countries including England, Germany, West of France, Scotland to name a few.