We started off with a 9ft (green canvas) CT with awning which with 3 teenage kids was a tight fit but we only slept in it. Our son would take his 3man tent on longer trips and stay in it away from his sister's.
We have now graduated to 12ft (maroon canvas) CT with awning & annexe(only use annexe for longer trips) and have heaps of room. We got the bigger one not only for more room with the kids now but when they all go we will have heaps of room for a table & chairs in the tent when it's just the two of us touring around in our retirement.
We havent had any issues with fitting into CP's with either CT. The Big 4 we stayed in at Xmas had huge grassed powered sites and we fitted the new 12ft CT with annexe on with no probs.
I would definately get them to set them both up for you to see size and ease of use. Our first CT tent was only 9ft and was custom made but was harder and took longer to put up than our new 12ft CT tent but the awning on the new one takes longer to erect only because of the spreader bars as the 9ft didn't have any.