I'm surprised by this question - a bit like the old length of a piece of string!  You need to find a picture, at least, of a similar size of GCR shed you want to model.  A drawing from, say, the NRM would be better.  However, just from photos of an ex-L&YR shed, you could try 6-8' for the outside pits and 3-4' for the inside ones.  Think about access for the shed workers with the doors closed.
A bit of research can be more fun than you might think.  :0)