Actually, it has everything to do with the camera or lens size. Most mid range cameras like full size DVCAM would take a 4x4 filter. Smaller cameras can get by with a 3x3 filter. Our HD camera with a 4.7mm lens takes a 4x5.6 filter. The prices rise with the size of the glass. There is plenty of good glass available on E-Bay. Most rental houses carry a lot of 4x4s. 3x3s are usually bought by the semi-pro types who know they need a filter but don't have a big rig to put it on.
Thanks Steve. I take it that the standard and most useful generally is 4x4. Is that grossly too big for HDV cameras? (I don't have an HD/HDV camera, and am waiting for the sweet spot to develop, probably DVCProHD.)