FWIW, I filled my APS-C sensor (maybe 90% of sensor height) with the supermoon the other week, using 1000mm f/14 (250 f/3.5 x2 x2).
Don't go too overboard with length, with that tight a framing the moon moved out of shot in about 30 seconds. (It was kinda freaky, watching it move on 5x live-view that quickly).

I know the sun's probably smaller than a supermoon, but 800mm or so is probably long enough for an aps-c (1200mm FF or so?) if you want to get a nice border around it like that palm-tree shot, 200mm x2 = 400mm and a bit of cropping on a 7D should be just nice.