Organisers will sell from a floor plan. Logically the earlier you book the better the choice of stand type and location you will have.
You will be able to see how many stands they plan, what the configurations and features are and where your stand will be positioned.You may wish to think about where your competitors are located, where the feature areas are and what you plan to do with your stand.
Allow enough room for any products as well as your own staff and visitors. You may wish to think about storage incorporated as part of your stand and any private meeting areas or hospitality that you wish to use.
If you are incorporating any activities on your stand then you will need to make provision for standing room and for any observers as well.
Try to plan for the number of open sides available and ensure that your furnishings do not obstruct and entrances and exits from your stand.
Most stand designers will be able to advise you whether the space you have reserved will be sufficient for the activity planned.