I agree, it's best to read as much as you can about all the options you have available before pulling the trigger.
If you have the possibility to listen to a few of them, that would be even better and I'd highly recommend you do that.
Regarding RP8 I will only say this: make sure you don't go over board for the sake of bass extension alone.
Sound quality between RP8 and RP5 and 6 is largely identical. Only thing you have extra is more bass response, generally speaking.
And there is an inherent risk about getting too large a speaker - namely room placement and room acoustics. Larger monitors are more picky about this and often times the bass can become overpowering and not very nice sounding because of reflections and reinforcement from the room at certain frequencies.
They generally need solid stands and lots of room to breathe to sound best, which is not always doable and not cheap either.
You do have treble and bass filters on the back of most monitors, so you can cut away excess frequencies and help the monitor blend in better with the room.
But if you do that with an RP8 you'll be throwing away most of the reasons you'll be buying it for, compared to it's smaller siblings.
Now, even RP5 has absolutely top notch extension for it's size and it may be the most extended in the bass compared to the other 5" monitors in this price range. They will be perfectly adequate in that regard.
And RP6 adds even more extension and authority in the sound and, IMO, would be the best compromise between the two - a 5" and a 8" monitor.
So that's also worth considering.
At any rate you should go ahead and read as much as you can find about your options so you have the best overall perspective before making your final choice.