usually a 10 x 4 = 5x4 x 2 roughly, occasionally 3x4 + 7x4
When i see pics of people's kennel set ups (no offence to anyone) i always think the kennel part is tall and narrow, so you can't effectively use the full height by putting up shelves etc.
But i a shed you can have a good sized shelf or two, or a hutch and shelf etc. There is space to sit in there with them, move around when you are cleaning out. Obviously a covered aviary is the best idea, but even if the weather is horrible and they dont go out, a shed is big enough for some running around, playing and binkying.
You can also arrange a shed + aviary on a right angle or off-set next to each other etc, however they fit into the space, whereas a kennel is a fixed rectangle.