Depending on the bow/ release combination my draw length is 28.5 - 29. I shoot a 27.5 arrow out of all my set ups.
I used to shoot 28.5" arrows on my 29" DL, but my new bow is 30". I'm right between spines with my DL, so I'm going to go with the stiffer spine, but add a bit of length, 30" or 30.5".
most places will help you fit and cut arrows to what size you need. every body size is going to be diffrent depening on bow and equipment.
I used the Pinwheel shaft selector free trial to choose the correct length that fits my bow/DL/arrow spine/fletching/insert/tip weight etc. http://www.pinwheelsoftware.com/ #70 HeliM 28" DL FlexFletch FFP360 50 grain inserts .300 spine = 29" arrow