Stock speakers

I am tryin to upgrade my speakers in my 03 alero i already got a new head unit with the right wire harness.What speakers do i need for the front and the back and take some recommendations too thanks

i'll give you a little bit of info on my experience with switching my speakers that may just help you out. i have a 03 alero 3.4 and i love it alot buttt the stock system sucked big time. i had a whole bunch of stuff from my old car that i ended up putting in and let me tell you, it sounds pretty damn awesome, like being in your own personal concert to say the least! here's what i did. i went with all boston acoustic speakers in the doors and rear deck in my car with there mid level equipment (which is actually 20 times better than most companys top notch line up's) called the SL series, they incorporate kortex dome tweeters with them which make highs sound silky smooth. the stock opening in the door is a 4x6 butttt that was too small for me, plus boston didnt make anything in that size in there SL model lineup, so what i actually did was took my 6 1/2 speakers i had in my previous car and instead of mounting them directly in the facroty location, i mounted them on the door pannels instead, right in front of the speaker grills. after removing the old speaker and mounting bracket for it, i put the door pannel back on with no hastle, and it makes things much louder and clearer with that larger speaker in there too! for the back i had a little trouble, im not exactly sure the exact size of the holes but if im to guess there 6x8's. nonetheless boston didnt make any that size AGAIN for there SL model, but with a little tough love i fit a pare of 6x9 in there and they went back in with a little force tight and secure. but if you decide to go with a diffrent brand of speaker make sure more than anything else that the magnet on the speaker is smaller than the actual dimentions, because if not your gonna have a hell of a time getting them to fit. also mine had the tweeter assembaly up in the door pannels and i had a extra pare of crossed over tweeters laying around so this is what i did. i ran the other tweeters right behind the door opening pointing at my head twords the back on both the passenger and driver sides, it makes a diffrence! then i split the line from the crossover and ran a new line to my already existing tweeters to give them protection from the high levels of curret provided by the new head unit. but my friend if i might suggest something, if you really want not only loud sound from your new speakers but also precise sound quality, i suggest amping all your new speakers except the tweaters, leave them hooked up to the new head units build in amp, but all others should be amped, it makes a massive diffrence! if you need any more info on anything let me know, i know every in and out there possibly is to these cars audio systems