Both the regular and extended sizes are 40mm wide and 90mm high. As the name suggests the extended version is just longer than the regular version at 210mm versus 150mm. The regular Top Tube Pack holds 0.5L, while the extended version holds 0.8L.
The Top Tube Pack sits on top of the top tube and can be mounted at either end. One velcro strap holds in to the top tube and the other either holds the steerer tube or the seat post.