They should be 5x8 speakers. And there are a few companies out there making component sets with 5x8's if you want to just drop 'em in.
You can put 6.5" components in there too but you'll need to do some trimming to shoe-horn them in. The amp will depend on the speakers you choose, so I'd pick those out first...... the speakers also contribute the most so don't be afraid to spend the $ for good ones.