Largest shack that I still need, that's a good question. I think, without checking my atlas, the bigger ones have all been done in the process of track bashing. I might look into this, in terms of the number of services calling I'd say the biggest one I need would be somewhere like Brentwood. First one that comes to mind at least, probably others that top it
Smallest I've had in terms of size will be Gilfach Fargoed, very closely followed by Dilton Marsh. In terms of service though, smallest would fall to numerous shacks on the Heart of Wales I'd say. Llanbister Road would be just one contender there, it and Llangynllo both serve some very remote and tiny places after all.