Just as an FYI, rattling doesn't necessarily mean you have to replace the speaker. But in case you decide to go ahead and do it anyway, here are my answers to your questions:

As for what size, I believe the '97s have 6.5" speakers in the doors. Hopefully someone will confirm that, but I know I had a customer come through about a month ago, and he had 6.5" speakers in the front of his '97 ESi(I pulled his door panel off to be sure). My '95 has 5.25", so I'm not 100% sure when the switch was made.

As for what kind... that's really a matter of personal taste. You might absolutely love the way Kenwood speakers sound, and I'm not all that fond of them. My personal favorites are the Boston Acoustics Pro Series, but someone else might be a diehard Infinity fan, and think the Kappas sound better than either Kenwood or Boston. A fourth person could think we're all wrong and beleive that Pioneer speakers are the best on the market.

Your best bet would probably be to go to a good stereo shop and ask to listen to some different brands in 6.5" components, and pick what you like and can afford.