I have two 6 1/2's and two 5 1/2's in my doors. What size amplifier would suffice?

 

Find the total RMS wattage of all your speakers. Match it or exceed it with whatever brand amplifier you so choose. And make sure it has 4 channels. That's it

 

It doesn't need to be 4 channel if 2 of the speakers are in the same door (or not, really) Just make sure the amp can handle 2 ohms per channel. or you can do 2 2 channels for better separation. one for left, one for right, or front/back.
But really, what we need is what you want out of your speakers, how you have it set up, what brand of speakers...
What tires are best for my ride?!? I say MT super swampers!, but are they best on your audi?....
Please give us a little information. We/I will be more happy to help.
What you have(hu, speakers...), what kinda sound do you want( clean, loud, both), what music do you listen to the most of, and most important, What kind of budget are we talking?