How much space is governed by what you store. Space hungry items should be avoided like Cycles, Mattresses, Garden furniture etc. As a tip if it can live outside, keep it outside and treat your unit space planning like a game of Tetrus.
If you are unsure, please pop in and see us. We can more often than not get you into space in less than you first imagine. This will save you money and also ensure that you only pay for what you use.