If You have the Alpine system, I honestly do not see the need to change the door speakers. I think they rock, they are clear, and are loud.. What Really Sucks are the woofers.
Remember, They use 2 woofers ( One on each side in the Back. That cover in the middle is just a ventilation, theres no woofer there ( just a vent).
The Woofers are the size of regular speakers. They are not even Alpines, ( They Are Audiophiles and they SUCK!!!!!!
I Swapped the woofers from my 98 that had JBL WOOFERS and they had more punch ( Biggers cone, Larger magnet, less distortion).
Really, If I were you, I would find stronger set of woofers with a separate AMP, or you can put a BOx set-up back there and give it a try first before removing the door speakers.
My Opinion!!
In the case of speaker sizes, I believe the woofers are 5 X 7
The Door speakers are circular, don't know exact size though