Today's runway 08/26 at Alderney is 877 metre long (277 metre longer than Scilly Isles/St. Marys) and it's more than long enough for up to Dash-8-100 / 39 seat sized aircraft. The problem at ACI is the width of runway 08/26. The surface is a mix of asphalt and grass (UK AIP says total width 23 metres, 2.5 metres of outer sides are grass and then the alphalt width will only be 18 metres). St. Marys has 23 metre with asphalt and that qualifies for Dash-6 (Twotter) operation, but isn't enough for Dash-8-100 where you need 30 metre wide runways.
I think the taxiways (12 metre wide) and the apron too need to be upgraded to handle larger aircraft than the Trislander. The position of the taxiway Bravo may also have to be moved westward since the bend today is so close to the shrubs and other vegetation (at least it was so the last time I flew in to ACI).
In other words, ACI has to be upgraded to handle larger aircraft and in most cases it is more difficult to widen a runway than to lengthen it.