Well the 4x M2 is 8.75 inches long and some have posted in the past who have attached a 4x to their carry handle and thought it was too awkward. When I got my 4x M2 I was surprised with how big it was, although I think it has to do a lot with it having a 30mm tube. I have it attached to a flattop and it works well in that configuration. Haven't had the scope long enough to truely determine how robust it is and how well it keeps zero since I have it on a QD mount that I remove each time I transport the rifle. Still have a lot of fiddling to do with the scope, mount, and rifle before I could give any real insight.
I would say that if the scope is too big, you could always get a flattop later down the road and transfer it to that. Still think a leupold or nikon in the 2-7x range with 1" tube would be a pretty good choice although it would not look very tactical. I would lean towards the nikon due to price and low recoil of AR-15, plus nikons have great glass, possibly even better than leupolds.