The width always refers to 2 numbers given in mm.
This is where the strap is fixed to the watchcase.
There is usually a difference of 2mm between these 2 measurements.
For example: 14mm (at the case) / 12mm (at the buckle)
The length always refers to 2 numbers given in mm.
The part that carries the buckle
For example: Size L strap: 85/155mm = 85mm for the shorter part and 115mm for the longer part