An interesting survey from Mamutphoto a while ago...

Tiny (35mm and medium format) : 11 - 10%
Small (4x5 to 8x10) : 66 - 60%
Panoramic (5x12, 7x17, 8x20) : 11 - 10%
Medium (11x14, 12x20) : 10 - 9.1%
Large (14x17, 16x20) : 9 - 8.2%
Mamut (18x22, 20x24) : 3 - 2.7%