I've been chasing bluegills with the fly for a long time and after thousands of personal encounters I have these indicators of size:

Dink: Anything a hardware fisher would have the same size of in his box with treble hooks hanging off of it.

Little Brother: Bigger than a Dink but smaller than a "Whoa". Usually you utter "go let your big brother know I'm here" when releasing one.

Whoa: Anything I have to put my rod between my legs or better yet on the ground because I need two hands to handle. Easily identified by yelling out "Whoa..." so all those around you will look and admire what you just tempted with your meager offering.

Moby: Anything requiring someone come help me because I don't want to lose the rod, risk breaking the rod, or I need a net.

The above words have been carefully selected so they can carry over the water so all those around you can congratulate you (or in the case of a dink) take a photo prior to release.