You will need to measure the:
Wheel size (Diameter) do not include the flange when measuring.
Track Gauge ie 9mm = N, 12mm = TT, 16.5mm = 00
Wheel centres the space between the wheels from axle to axle.
From this you can be sure you are ordering the right motorbogie.
There are smaller motor series 1015, 1020, 1024 which measures; minimum length of motor gearbox and flywheel from 30mm for the 1015, 35mm for the 1020 and 39mm for the 1024 height from centre of axles for the 10 series is 20mm, width 10mm.