Sennheiser recently discontinued this headphone lineup. The only one they still sell new is the HD200 Pro which I’m not a big fan of since the cable is permanently attached.
As far as I’m aware they never published a driver size spec for these so they might be smaller than the “standard” 40mm.
But that’s okay, driver size is not a guarantee of sound quality. It’s a fun thing to know, but so many other aspects of the design also contribute to the sound of headphones. Knowing the driver size doesn’t necessarily tell you anything about performance/reproduction characteristics.
Think of it like speakers. Big speakers in a badly treated room can sound quite poor, and tiny speakers in a proper near field setup can sound great. In headphones, the design of the cups and the pads stands in for the room.
Thanks for reading!