Atlas is nice but there's a new smaller version of the LRA that's much nicer for about the same price or even a bit less. The LRA is the best, period, but it's big and expensive. Atlas is nice if you get the right one. Harris is fine unless you are invading a mountain country and plan on being there a while with no access to supplies. They are fine for lighter rifle. Get an S model so it'll swivel and be sure to buy a "Pod Lok" --it's like $11 and gives you the ability to lock the can't. Be sure to get the segmented legs too. As for attachment, get the regular stud mount. Then get a stud base for the bipod. Please don't get a good bipod and cheap out on this part. KAC, Era Tac and DD all make good Keymod mounts. I just bought the Era Tac as it was what they had. $55 yeah, but it's a damn nice part, every bit as nice as KAC.
Oh, KAC has a bipod too, has all the shit you could ask for but it's pricey.
DO NOT get counterfeit or fake shit. It hurts us all.
9-13" is the most common size, it's what's used on all my AR's that use Harris bipods. It will generally balance on the grip and bipod with loaded mag, just enough room for mag to not hit the floor. No problem prone provided you have sand, sand bags, a shooting bag --hell, there's a million ways to deal with that.
Never had a use for the long one, don't even own one. Only own two short ones, one on a .22 and one on an 11.5" SBR (which would have got a 9-13" but it's just too long).
All my future bipods will be the new Siinclair models though, the smaller version of the LRA. That's the one we've all been waiting for I think. $225, it costs as much as the cheaper Atlas but has all the perks. And will mount directly to your rifle, no mount needed.
You can get bipods that have been upgraded by various companies with their gear too. Larue comes to mind, they make a line of modified Harris bipods. If these float your boat, they are worth looking at too.
FWIW, I use an Atlas on my M110 but it's because the new Sinclair wasn't around. I'll likely swap that out, put the Atlas on a 6.5G. But I would have been happy with the right Harris too. They've never let me down, they're just not made for use on 20+ lb. rifles.