Size itself is not an issue.
You will need planning permission if:
– your proposed deck is over 300mm (roughly a foot) high
– where the outdoor decks or platforms (including outbuildings) cover more than 50 per cent of the garden area
– where the deck is situated within 20 metres of a highway
– if the structure would affect the amenity value or privacy of neighbouring properties
– if the deck is attached to a listed building or situated in a conservation area or National Park