My draw length is 28″, what should the length of the arrows be with broadheads?

There really is no set length that the arrows should be. Typically, people who use fixed-blade broadheads will leave the arrows long enough so that the broadhead sticks out in front of the riser. But as long as they aren't too short to be on the rest when at full draw, you should be fine. To put a set number on it, I'll say the arrows without a broadhead, will be about 28" long.