As a general rule, if I'm making a model I try to keep the polys (in the gmax poly counter) down below about 60,000.  MakeMDL seems to have a problem with anything over about 80,000.

Just remember that you only need as many polys as will fool the eye.  Never use a 32-sided cylinder when a 12- or 24-sided one will work.  Keep it simple, but show the detail.  Many small details can be shown by simply painting the aircraft carefully.