That should be more than enough, I always launch my MPR stuff and even my Mean Machine off a 6ft (I believe) rail, a LOT better than using a rod. The 1515 is about as big as you would ever need, 1010 is good for probably up to J or K.
The rule of thumb for rails is the same as for any rod. You should have about 44fps off the rod/rail in winds up to 5mph. Like the often quoted 5:1 weight ratio, it is only a starting point. Your mileage will vary.
The original standard BlackSky rail is 48" long. I've flown lpr to hpr off this rail.