A brief note on specimen size abbreviations:
Size Definitions  
Generic Dimension
Maximum Dimension
in inches
Maximum Dimension
in Centimeters
Thumbnail (TN)
1 1/4
3.1
Miniature (Min)
2 
5.0
Small Cabinet (SCab)
3 3/4
9.4
Cabinet (Cab)
 >3 3/4
 >9.4


Or for a more "gut feeling" approach...

THUMBNAIL: fits in a 1.25 inch square "Perky box" (invented by Willard Perkins to hold small rocks)

MINIATURE: simply a rock that can be squeezed into a 2-inch square box

CABINET: A hand-sized specimen larger than 4 inches (10 cm ) in a dimension

SMALL CABINET: in between Miniature and Cabinet...you can hold it in your palm

TOENAIL:a new division in Tucson show judging - basically they are just barely too big to squeeze in a tn box