[Semont]

I often hear of dynamic drivers with stated sizes but this never occurs with planar manetic drivers.
 
What's the reason behind this?
 
Does membrane size for orthos make a difference in sound (I'm sure it does)?
 
Is there a standard size?
 
Is the placement and location of magnets instead, either on one side or both sides more important?

 

[TMRaven]

Audeze lists the diaphragm size of their headphones on their site.  I don't think Hifiman does, but their diaphragms span the entire diameter of the cup minus some thickness of the cup itself.
 
Planar Magnetics operate differently from dynamic cones.  The dynamic operates in a piston-like fashion, and excursion is higher.  Planar Magnetics operate in a trampoline like fashion, with very minimal excursion.  There is an inherent difference in sound, but it's not extreme.
 
 
I don't know what you mean by standard size.
 
One sided vs double sided vs asymmetric arrangement of the magnets is important.  It goes quite a ways into effecting acoustics and diaphragm control.