The specs do list the panel dimensions as 40" X 9.3". Looks like the panel starts right about where the cabinet's top ends, so I would say 15"-17", but that's just a guess. Also, the cabinet is sloped a little.
Thanks very much Mickey for the 411 on the vista's . I checked your system and it looks awesome bud I just ordered these speakers and its going to line up with my entertainment center height wise thanks to your confirmation