Layman's or red neck's approach . . .
Nominal Bar Length is typically determined by measuring from the edge of the bar bolt slot to the tip of the bar.
To determine the pitch of the chain, measure the length across three chain rivets and divide by two.
To determine what chain fits a bar - first look for information imprinted on the bar? Sometimes it is metric.
Try a chain of known pitch across the spur or rim sprocket on the saw and it must mesh all the way around to match.
Try a chain of known pitch across the nose of the bar - again, it must mesh all around the nose.
Bar rail grooves have different widths like .043", .050", .058", and .063". Try some known chains in the bar rails to check fit?
The chain driver links can be measured for width with a digital caliper? But there are often numbers on a driver link that can be referenced to the nominal thickness.
Looking for a bar to fit your chainsaw? First compare the mount part of the bar of question with a known bar?
Points to consider? Mounting slot width compared with the mount studs on the saw?
Oil hole location?
Pitch and gage?
I go to a website like Bailey's in the bar advertisements and see what saws they claim fit what bars? Sometimes there are mount diagrams?