I installed 5.1/4 MQ Quart componet speakers in my front doors and they fit
perfect.  You could fit up to a 6'' in the door as long as its either a
really thin setup or it's just a componet speaker ( just the woofer and not
the tweeter, it mounts somewhere else)

Chris Winesette
American Advertising
Director, Creative Media
chriswinesette@aol.com