If you go red with RRRR 2x2 that mean RR for main and RR for FoC. That's what I have now. The result is a brighter Red flash on clash. Instead of the saber shimmering dimmer as a PC w/o FoC would do, it shimmers brighter when the other two dies come on. Personally, if you were going red that's what I would go with. I'd be concerned about run time with full power RRRR on all the time. Bright yes, but I dunno how long it'd run before you have to charge it for a couple of hours.
The color mixing and FoC depends on what LED you choose and what you want for the main die. If I were to go purple for example, I'd go RGBW to have RB for Purple and GW for a greenish/white FoC. It's a very dramatic contrast. Same for RGGB, GG for main green blade and RB for a Purplish/white FoC. That's a awesome effect too. That'll be on my Qui-Gon Jin for certain. No debate there.
RGGB can also be used for a nice Cyan bin1 color. GB or GGB for the Cyan and R or RG for an orangish/white FoC.
Straight BBBB 2x2 wold be the same effect as the red with a brighter blue flash. And for blade length, it will very from person to person, but keep in mind that about 1.5" goes inside the blade holder too. I have 36" blades and I like that length, but that's personal preference.