Probably best to get 6.5" for the back and front as you will have to cut the parcel shelf to get 6x9s in there.
Factory, R33 have 6.5" so they will almost just drop in.
I'd say maybe a 4 channel amp to run the speaker for awesome sound, 6.5" splits in the front doors (tweeters), 6.5" speakers in the parcel shelf and the DVD head unit work work mint.
Not sure on how much a PIONEER double din screen is, but brand new the single din ones are about $1,000 - $1,400 and 2nd hand about $650 - $1,000.
I have seen 1 x double din head unit for sale, brand new selling for about $1,300 actually. I will see if I can fidn you the link for you.
Hope that helps.