Components are normal speaker with a specialized circuit to route specific frequencies to specific drivers. A typical component set has 2 mid-bass drivers, 2 tweeters and 2 crossovers. The crossover takes the signal from the HU (or the amp) and then route the lower frequencies to the mid-bass and the higher ones to the tweeters. This helps create a better sound stage. Also, for best sound reproduction the mid-bass and tweeter should be placed at an appropriate distance to each other, i.e., neither too close nor too far.
My suggestion would be to get 6" components for best sound up front. If 6" is not possible (without any mods) then go for 5.25".
freakdude.