http://en.wikipedia.org/wiki/Wagon_Wheels#Size
There have been many debates amongst fans of the biscuit about the size. Wagon Wheels have supposedly shrunk in size as time has progressed, Burtons Foods LTD have denied this. It has been suggested that the supposed shrinkage is due to an adult's fond childhood memory of eating a Wagon Wheel held in a much smaller hand.
The original factory which produced the biscuit in Slough during the early 1980s were transferred to an updated and modern factory in Llantarnam in South Wales. The original factory in Slough produced the biscuit with crinkled edges rather than the updated smoother edges. This caused the overall diameter of the biscuit to shrink slighty, but not as much as fans of the biscuit believe.
Also, although the UK Wagon Wheel has barely shrunk, it is still noticeably smaller than the Australian equivalent. The diameter of the Australian version is measured at 88 mm which is 14 mm larger than the UK version. Even though the Australian version is wider in diameter, the UK Wagon Wheel is notably thicker by 4 mm.
As of October 2006, the Canadian Wagon Wheel measured 70mm x 17mm.