Installing speakers - '91 Corolla

[Dirtjump9]

I'm going to finally go ahead and get my sound system done up in the near future. The only thing is that the front speakers are mounted in what seems to be a pod kind of deal. I don't think there are any speakers that are going to be a direct drop-in to the stock setup. Does anyone know what size speakers can be stuffed into the stock front speaker locations in the '91 Corolla?
Another thing I have in my head is custom speaker placement. I though about getting two small tweeters and have them installed in the dash on each side. Since the car came stock with two front speakers in the dash kind of aiming down at your knees and two rear deck speakers, I was wondering if it would be worth it to do a custom install with some mids in the door somehow or something. Just a though... It's just that I don't know how good the sound quality will be with the stock speaker placement...
Also, I am debating on weather to run rear deck speakers with the system or just get two nice front speakers along with a woofer for the trunk. I want the sound to sound very good in the front seats but since I always have some passengers in the back, I don't want to neglect them by depriving them of mids and highs and drowning them in bass. Anyone have any ideas? I though about getting some cheap 5 1/4" mids for the rear deck and maybe rig in a potentiometer so I can control the loudness of the rear speakers but not affect the subwoofer.
I'm a newbie with car audio and I'm trying to grasp as much knowledge as I can before I go to the audio place and pick out what I want.
Thanks for your help!

[hiace24]

Front speakers- In most cases speakers mounted in the dash are only about 4 inch. And even if you manage to stuff bigger speakers in there they will sound like crap cos theyre facing down. Youll want to put your speakers in the door.

[Dirtjump9]

Thats what I though.
I though about mounting them in the door but I don't think it is readily possible. The doors seems like they aren't deep enough to properly mount speakers and I am on a low budget so I'm not too sure what I am going to wind up doing.
I have read that it is best to build your system in parts and begin by augmenting the equipment you have but the problem is that it is so old and cheap. The head unit is crap and only plays casettes and AM/FM; the rear speakers didn't even work; the front speakers have holes in the cones; and there is no real practical reason to augment what is in there because it is all so obsolete.
But yeah I am going to see if I can pull the door panel off later and do some measurments and observations.