Paul has many good points. I just thought I would muddy up the water a bit.

Many of todays rests actually mount behind the riser or grip of the bow, in essence being an overdraw. Examples being the ever-popular Whisker Bisuit and Muzzy Zero Effect. Because of this, you can shoot arrows that are shorter than your draw length, given that your riser cut-out will allow this. Both the NAP Quick Tune 4000 and my current QAD Ultra Rest mount about an inch behind the riser. My draw length is 28" yet I shoot 27" long arrows. The end of the arrow shaft is about an inch in front of my rest and the field point or broadhead is in the middle of the riser. Most newer bows will allow you to do this because of the larger cutout in the riser. If you're using a fixed broadhead with a large cutting diameter, you may not be able to do this because of clearance problems.

There are two benefits to being able to shoot shorter arrows.
1 - The shorter length will increase the spine of the arrow allowing you to shoot lighter arrows.
2 - The lighter weight will increase arrow speed.

It used to be common for people to have their arrows extend to the front of or an inch beyond the riser. Two examples quickly come to mind.
1 - Not enough clearance. Risers did not have large, cut-out windows. Their was generally just enough room for the shaft and fletching to get by.
2 - Safety. There were not a lot of full containment rests before a few years ago. Nobody really wanted a broadhead tipped arrow mounted behind their hand. The possibility of it falling off and going through your arm wasn't very appealing.

To answer your original question, the length of your arrow depends on your personal setup. The arrow could be longer, shorter or 28.5 inches. Get your bow setup the way you want it and then go to your local pro-shop to have them measure some arrows for you.