I have posted this before, but since it is now buried in the archives I thought it would be a good time to share my rules for sizing lobster with you again.
If it looks short --- it's short, leave it, move on.
If it looks borderline -- it's short, leave it, move on.
If there is the tiniest tinge of doubt in your mind -- it's short, leave it, move on.
If it looks legal -- measure it before you bag it.
If it’s bigger around than a fat beer can -- bag it. (a.k.a. Foster’s rule)
If it’s two hander -- bag it.
Bugs seem to shrink when you bring them to the surface -- measure them all again before you transfer them to the holding well. (a.k.a. the inverse pressure effect)