Measuring for replacement BB
 

I measured the distance across the frame from edge to edge of the cylinder that houses the BB. I got the equivalent of 73mm. I'm not sure about that second measurement though. When I looked up the replacement, I got choices of 73 x 113, 73 x 118, 73 x 122. My guess is that number is the length of the axle that the cranks are mounted to. If so, mine was 2 mm off from any choice I found. I believe mine measured 4 11/16. Do I go greater or lesser in length? I have the Diamondback Edgewood. I'm going with the Shimano BB-UN55. I'm also replacing the crankset and installing a Shimano CS.

Sorry, but I don't know all of the bike parts' names yet.

If you are going to replace the crankset then you will need to get the correct BB for that crankset. What you have now may not be correct for the new one.

Probably the safest bet is to remove the current BB and see what it says. It almost certainly has the size on the shell.

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.

New crankset. There's the key:

While there are other things that could become factors, the single MOST important factor in choosing a bottom bracket spindle length is the crank that you are planning to install. Usually you can get a recommendation from the crankset vendor. Even then, however, sometimes they'll throw you a curve and recommend something like 113/118. If they do that, check out your bike's rear dropout spacing. Use the shorter spindle length for 130mm dropouts and the wider length for 135mm dropouts to sweeten your chainline.

As noted, the type of crank you are getting will determine the size and type of bottom bracket you need. The UN-BB55 is a square taper spindle and requires a crank with that interface. These are still available but there are a lot of different and newer styles that require a different design bottom brackets. Buy the crank first, then choose a bottom bracket to match.

How much clearance will the new crankset have between the inner ring and the chain stay is another factor besides chain line (as in you need to have a few MMs of gap). So one might mount the new crankset on the old BB and see what chainline and clearance exists then from that choose a new BB that corrects these dimensions as needed.

As to why some cranksets have two BB spindle length suggestions- it's often for how the ft der works. A seat tube which is larger will position the der cage further out from the frame's centerline. So to not have the der cage no be able to rub when in the lowest gear combo the crankset needs to be slightly further out also. If the ST were of a smaller diameter then the cage inward range limit will be closer to the frame's centerline and a shorter spindle can be used.

So you see there are a few factors that are almost only "fit it and see" aspects of best function. Andy.

Thanks for the responses, guys. Much appreciated.