are you going to be running the speakers off an amp?
im running alpine type R splits in the front and 6" type R in the back. Splits are 110w RMS and rears are 100w RMS.
Standard r33 speakers are like 10w RMS, 6" or 6.5" cant remember. I can fit the 6.5 Inch cone easilly in the standard spacers/brackets wateva.
If your going to run an amp, get 6" speakers, something with a high RMS and them run them off an amp that will support them. Will sound better in the car, too much bass might sound 'fully sick' but deep down you know its shit!
So for example, if your going to run the speakers i have, you need a 110w x 4 amp to use the full RMS potential.