Before you take apart the old make note of a few dimensions for reference in deciding which replacement fits best. Chain line, inner chain ring clearance WRT the RH chain stay. Whether the old BB spindle is a symmetrical or an off set type (are both ends the same length or does the Rh side extend out further then the LH side). And of course the BB shell threading/width.

Then you can Begin to make some assumptions about a replacement. But remember that even pros find themselves going to a second and different sized replacement at times. This is where your relationship with the place you purchase the BB from can change your experience. A good relationship with the LS you deal with will make the possible return and purchase of the second BB go smoother. A poor or lack of relationship (say as an on line purchase) might make the return of the not quite right first BB not so pleasant. Andy.