yep! what yagz said 6.5inch splits in front, 6inch in rear and 12inch sub is a good all round size. just get decent quality speakers and have enough power running to speakers and sub, a big 4 channel amp for speakers and a seperate amp for the sub should do the job. and a good box to suit the sub. and it will sound pretty good.
As a recommendation, I have 6.5" Earthquake speakers in the front and 7 x 10" Kenwoods in the rear. I don't have a sub, but a lot of people think I do. The earthquake 6.5"s have really good bass and are also very clear and have very good mid-high. I would recommend them. The standard size for the front is 6" but the 6.5's fit without too much effort and are worth the hassle.
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