I have an HMR but I don't have the HMR stock on it, so I'm not sure if 14" would be too long.
My KRG Bravo has a 12" ARCA rail on it and it overhangs the front of the forend by 2". That's about the most I'd be comfortable with, as anything longer might give you a bit too much flex if your bipod was clamped to the end.
With that said, I switched from Harris on a sling stud to an Atlas CAL on an ARCA rail and it's one of the best purchases I've made for my rifle. Also, Area 419 is top notch and their customer service is the bees knees.