I wouldn't use any sites that only accept club members, it's a joke.
Any normal site including the ones that are club recognised have never checked my tent size, when booking, just say its 6m long and then put the lot up, he who dares wins
Just done some "test bookings" for next weekend and had no problems.I tried 3 random sizes--8x4mt--5x5 mt and 10x5 mt. The number of available sites did decrease for the largest size tent. I am a member of the candcc and have never had any problems booking a larger tent in.They normally have the dimensions listed on their computer system when you tell them the tent model.With a larger tent you may find your choice of pitch type limited eg with ehu. If there was a limit on tent size of anything like 3mt--I would no longer be a member.As it stands--I think membership is good value for money and gives you access to some good sites--Its just a shame about the man in the hi-viz jacket
A couple of years ago we were in Spain when a young couple came on with a massive trailer tent. It was so big it filled the pitch and they ended up asking everyone around if they could sit on their pitch to get some sun!!! The laugh was they virtually only slept in it and went off out every day apart from early and late in the day asking about sitting on other peoples pitches!!!