If you use an efficient drill, like a nils, 6 inch is no problem. I would recommend a hammer drill, you will be putting a lot of load on the gearbox, and the hammer drills are built for that. I got a cheap Home Depot (Rigid) and it has drilled hundreds and hundreds of holes. I use up a 4 A-h battery most days hole hopping, and it has gone 2 seasons- like 2000 holes. If I ever have a problem, it has lifetime warranty- on batteries as well as the drill.
"Im a man, but I can change- if I have to ... I guess"