In addition to getting as much area as possible in the shed, maximise the ingenuity too. One of the cleverest ideas I have seen is arranging return loops outside the shed on covered stages. With bushes planted underneath these were not visually intrusive, and with the covers removable at one side there was an outdoor 'mainline view' of passing trains from a nearby seating area. It also enabled the layout plan in the covered area to be 'walk in' rather than requiring a duck under to get access to a centre operating well. (The builder was thinking well ahead to a possibly less flexible old age.)