The length of the arm has nothing to do with pressure or water flow. It is purely personal preference. There is no "right" height. Obviously, every person isn't the same height, so whatever height you put it will be different distances away depending on the person. Also, the ceiling height plays a big role. If you have 8 foot ceilings, the 6" might be perfect. If you have 9 foot ceilings, that would be a really high shower head!

In general I prefer the bottom of the rainhead to not be lower than 7' above the tiled shower floor. If the plumbing is already in place, you probably have a capped galvanized nipple stubbed out of the shower ceiling. That nipple screws into a threaded copper or brass union buried up in the ceiling. Let's say that union is "buried" 2" up into the ceiling. Let's say your rainhead is 4 inches "thick". Your 12" long arm will go 2" up into the ceiling, then protrude down 10" into the shower. With the 4" deep head screwed on the arm the bottom of the head will be 14" below the ceiling, or with an 8' tall ceiling it'll be 6'10" above the floor. Now a question...that 8' ceiling, is it 8' in the bathroom area outside the shower but less in the shower due to the thickness of the sloped shower floor? You could have 2"-3" of mud thickness at the drain, the 12" arm would then put the shower head 6'7" to 6'8" above the shower floor. That might be okay for you, but it'd be tight for me, I'm 6'4". So best advice is to take a couple of measurements in the shower and see what dimensions you're working with. Try to keep the head about 10" to 12" above the head of the tallest bather. Higher is fine.

My master bathroom remodel is nearing completion. Based on the salesperson's recommendations, II chose to put in a Hansgrohe Raindance S120 wall-mounted shower head on a Hansgrohe Axor arm, a Hansgrohe Select E120 hand spray (with 3 spray modes) on a hook, and a ceiling-mounted rain shower head. I have all the parts in hand, but have vacillated in choosing the ceiling-mounted rain head and extension pipe. I was planning to get the 13" round Hansgrohe C300 (with RainAir spray mode), but designer suggested the rectangular Hansgrohe Raindance E 360 (14"x7.5") (with RainAir spray mode) would fit the space better (shower is rectangular, about 3'x6'). I'm also looking at the Hansgrohe PuraVida 400 Air Shower head, which is 15.5" square and has a RainAir XXL spray mode. This model comes in white or chrome. Any experience with Rain Air XXL spray mode versus Rain Air spray mode? My ceilings are 8' high. What length ceiling pipe? 6"? 12"?

Agree with what Mizlizzie wrote. I try to keep the bottom of the rainhead a minimum of 7' above the tiled floor. You can mix drop arms. Most are universal 1/2" thread. Generic finishes like Chrome are pretty much universal, you can mix and match from most any manufacturer. If you have a designer-type finish then sure, it makes sense to stay with within one manufacturer's product line.