You have a few different "sizes" to worry about, as a new bottom bracket will need to fit both the frame and the crankset. The cups or cartridge body will need to be the correct diameter and threading to fit the frame. If it's an adjustable type bottom bracket, the spindle's races need to be spaced correctly for the frame's shell width. The spindle's overall length needs to match the crankset's design specs. The spindle also needs to be the correct type (and taper if applicable) to mate with your crank arms.

Most Shoguns I've seen or heard of were Asian bikes made in the '70s and '80s. If that's the case for your bike, it should be safe to make a few assumptions: Your frame's bottom bracket shell is probably 68 mm wide and English threaded, and you probably have square taper cranks. Being Asian, the taper is going to be JIS (as opposed to the ISO taper often seen on Italian components). That leaves one critical dimension to worry about: spindle length. The correct spindle length for you will depend on what crankset you're using. The safest way to figure this out would be to measure your existing spindle. Or remove it and look for markings that you can cross-reference.