Overall size: Its the thinnest gun I've ever seen, around 2 inches in length. And it is as long as the Raider without the stock.
Darts: Streamlines.
Pics:
Nerf Alpha Trooper CS-18 box, front.
Nerf Alpha Trooper CS-18 box, back
Nerf Alpha Trooper CS-18, without box cover
Nerf Alpha Trooper CS-18, contents
Nerf Alpha Trooper CS-18, fully assembled
Nerf Alpha Trooper CS-18, size comparison with Maverick
Nerf Alpha Trooper CS-18, size comparison with Raider
Nerf Alpha Trooper CS-18, rifle mode
(not recommended to rest stock against shoulder, but if you feel that it is comfortable, do it)
Nerf Alpha Trooper CS-18, with Raider 35 drum-clip and Raider's stock.
Clips and drum clips size comparison
(First from left, standard clip 6 dart clip. Second from right, Stampede 18 dart clip. Third from left, Alpha Trooper 18 dart drum-clip. Last from left, Raider 35 dart drum-clip, which is highest capacity yet)
Pictures belong to SG Nerf.