After I get my wheels I plan on getting a little better speakers, small sub, amp, ect. I have a couple questions?

What size speakers would i need for the front and back?

I really like the 6 disc cd changer, would I be able to run a sub off the stock head unit?

Do I have to put the sub in the trunk?

I want more power than I have now but nothing crazy. Any suggestions?

6.5 in the front, 6x9 in the back.
yes, get a line output conv. tap into rear speakers
you can put the sub wherever you want I removed my backseat and put it there
I want a new HO alt too

I would not put new 6x9s in the back deck......you will have tons of highs/midrange coming from behind your head. Bad imaging musically and very 1980s. I would upgrade the fronts with new seperates, leave the back speakers as is for rear-fill, add a small amp and sub in the trunk.....whaaalaaaaaahhh!!! Great sounding music.