Standard PC case fans are square in shape.
They are measured from edge-to-edge. I.E. an 80mm fan will measure exactly 80mm from end to end.
Fan speed is measured in rounds per minute (RPM).
How fast the blades spin per minute. Not really a useful spec.
Measured in cubic feet per minute (CFM).
A higher number means more air getting moved.
PC fans generally move about 20 to 70 CFM.
Generally measured in decibels relative to air (dBA).
25dBA is a quiet library, and PC fans are generally around 20 to 30 dBA.