I wouldn't touch a 10 x 4 shed, sounds more like a tunnel, You would be
better off with something like an 8 x 6


Why would you recommend not touching it? Is there something about getting
this particular dimension that makes it unstable (or something else). After
all, I just want to use it as a storage area for a oddly shaped part of the
garden (I'm also going to partition in the middle and have two doors).