We have been selling collector rubber ducks for quite a while now. Having said that, no proper accepted size classification has ever been standardized, nor accepted.
We have our own:
Micro: Under 1 Inch
Mini: 1 - 2 Inches
Standard: 2-3 Inches
Large: 3-4 Inches
Extra Large: 4-5 Inches
Jumbo: 5-7 Inches
