Attachments are scaled to the scaling of the unit they are attached to, provided you tick the "scale attachments" (or something to that effect, it's under the "art" fields near the top) field. IE, bigger unit = bigger attachment.
Other than this, I don't think there's another way to do this unless you make a unit with the model and scale that to what you want, then make a trigger to get the fake model to follow the "attached" unit.
Or just edit the attachment model.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.