Currently the largest is TCX A321 however they will base an A330 at STN next Summer for 6 weeks or so.
Plus TOM to Tunisia on Sundays will be operated by a LTN based aircraft next summer on a W pattern, which I believe will be a 757 but not 100% sure on that. You never know there may even be some kind of long-haul airline in the mix by then so could get a little more in the way of larger aircraft.