The "73" refers to the width of the bottom bracket shell on the frame, as you surmised. The second number refers to the overall length of the spindle, and depends on a number of factors including the number of chainrings (more rings require a longer spindle), whether the chainstays are bowed to accommodate wide tires (again, a longer spindle is needed to get clearance for the arm), offset built into the crank arms, etc. If your frame has a bottom bracket installed you can measure it or remove it and look for markings indicating those measurements. If you're planning on installing different crank arms, the manufacturer should have recommendations on what bottom bracket to use.