Ideally you measure the lenght of the pipe from the center of the sparkplug to widest part of the pipe. But most commonly used measure is from exhaust port to widest part of pipe. As Roman remarked the BZM is indeed basicaly a 40 or 50 cc size cilinder, so the distance from center of spartkplug to exhaust flange is more then for example a QD or Zenoah. So I measured up the lenth of these:
BZM: 41 mm
QD: 36 mm
Zenoah: 32 mm
So correction for QD should be 4 mm shorter (that is about 1/6 inch)
for BZM it should be 9 mm or 1/3 inch shorter
Shorter here meaning that if the lenght for a zenoah from exhaust flange to widest part of part 13 inch is the optimal lenght, with a QD it should be 12 5/6 inch and with a bzm it should be 12 2/3...
However as with any set-up you need to find the sweet spot, so the given lenght is just a starting point as the characteristics (powercurve) of the engines are very different.