Is there any maximum speaker depth i can have for installing 6" speakers in my lower front doors, like so it doesn't interfere w/ the window etc. any help is much appreciated.

 

IIRC thers is about the standard depth for a 6" speaker, IMHO try and find the BOSE kick panles, or make your own. I have seen too many people destroy door panels trying to install door speakers and it didn't make a difference in the sound quality. It's your car so do what you will, but investigate the options first.