You can go with 6 3/4 in doors and on rears on the GT hatch. I've changed head unit to Pioneer Premier that puts out about 20 watts RMS per channel as opposed to the stock unit which probably only generates 10-15 watts. The theory is the more watts to the speaker(up to about 80% of their maximum,not peak,rating,) the more efficient,clean and better sounding the speaker
I changed out the rears to Pioneer 1670's 3 ways, cheap at $79 a pair, and added an Alpine amp to amp all interiors. The fronts are still stock and will be changed soon, but I gotta say, the sound after amping is worth the effort and mods can be done with the excellent assist of this site and DIY at
http://elantragtclub.tripod.com/.
As for bass, a subwoofer is a must have for the punch and also makes your interiors more efficient by routing low frequency bass to the sub.
I'm using a multi-channel amp that allows for 100 Watts to the 10 inch sub which for rock/blues/jazz is plenty adequate. It's a matter of your taste in music and the bucks you have to spend.