If it is a line segment, just click on it with the Properties display opened. It will give the length, but you can't copy it. As MikeHalloran said about the pline, LIST will also give a copy-able .txt display with length, like this:
Command: list 1 found
LINE Layer: "0"
Space: Model space
Handle = 8b
from point, X= 35.5496 Y= 17.8966 Z= 0.0000
to point, X= 15.9339 Y= 19.0093 Z= 0.0000
Length = 19.6472, Angle in XY Plane = 177
Delta X = -19.6157, Delta Y = 1.1127, Delta Z = 0.0000
Remember, amateurs designed and built the ark...professionals designed and built the Titanic. - Steve