I just put up 2, You should stand or sit in shower & determine where you would want the bars. Maybe that would help with length question.
I got a short 12 inch but couldnt find a stud for at least a few of the screws. I ended up using wall mollies on a temporary basis. I put a label on it (bar) as a reminder "will NOT support full weight. I just matched the metal and style of what was already in each bathroom, stanless, brushed nickel.
CF