The size of your garden gym will, to some degree, be dictated by the space you have available and how you wish to equip and use your gym.
In most instances, the optimal size of a garden gym is 5m x 4m. This provides sufficient space for a range of equipment as well as an area to warm up and cool down. If space is more limited then a size of 3m x 3m is adequate for accommodating one piece of cardio equipment (a bike, cross trainer, runner or rower) and weights.
Something else to consider is the height of your garden gym to allow sufficient space for exercising and stretching. Under current guidelines, you can build up to a height of 2.5 metres without needing planning permission as long as your new building is less than 2 metres from your house or a boundary line.
If your garden gym is more than 2 metres from your house or a boundary, then it may be possible to extend the height to a maximum of four metres depending on type of roof chosen.