I guess the length would depend on how you plan to use it. In my case, I wanted the spray bar running the entire length of the tank. So compensating for the intake and some distance on the other side of the tank, I made mine around 30" for my 36" tank. I'd guess the holes are spaced out every 1.5-2". I started with smaller sized holes. If the spray felt too strong, I just enlarged them until I had a nice gentle flow running the length of the bar.
Oh, and I just took the filter-supplied short spray bar to Home Depot and found the best fitting parts while at the store. Kept me from having to make multiple trips.
No good suggestions on how to determine back pressure problems.