I have a roller door on my garage and the mechanism housing is 300mm tall and protrudes inside by the same amount. The drop on the door is 2m which I know for you is probably only half what you need but when rolled up there is not much space in the housing maybe get another roll on it at a push. At a guess to roll up say a 4m drop I would say you are looking at a housing at least 400 X 400mm taking into account that as the roll circumference increases more of the vertical drop is accounted for.

Richard