Most, if not all, sites will accommodate tents less than 5 metres wide and not more than about 7 metres long. Some sites however have different prices for small, medium and large tents and I was recently charged more for my medium sized 4 man tunnel tent than a friend was charged for her large 5 berth caravan which was twice the length of my tent.
It's not the size of the tent ,but the shape. As you say a tunnel tent is the best if you want as big a tent you can have without paying for the pitch next to you as well the C&CC lay down a maximum rectangular size and this is usually the guide to other sites.There is also a thread which gives you a list of sites that allow big tents
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dead horse
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donkey buyer