I'm finally breaking down and doing my lighting mods. I have a Warrior sport rack, so I'm doing four on the roof up front and two rear facing on the back. I'm also installing two on the windshield.

I plan on wiring the rear facing ones to the backup lights, by drilling into the housing. Since I'll be running the wires inside the bar, I won't need too much, but I planned on using flex-tubing for the ends (and to cover the wires from the lights to the bars).

My question is, would 1/4 inch tubing be too big? Has anyone else used it and if so, how big or small a tube did you use?

 

Split-loom size depends on wire size... wire size depends on electrical load; ie wattage of lamps to be powered by said wire/s. So, I need more info about your "planned light mod".
Additionally, I would not recommend running anything other than LED lamps if your plan is to power them "directly" (without relay) via the OEM back-up lamp circuit. I have had two devices powered "directly" from the B/U circuit; that have been working perfectly for the last two years. One "parking sensor" device; and two "semi tractor/trailer" style B/U lamps. These items pull very little amperage; and only operate as long as the gear selector is in reverse position.
You may want to consider providing a dedicated circuit for those rear lamps; so you can use them when the JK is parked. They will make great area/work lights for camp set-up, etc. Enlighten me... of the plan specifics!