The link Walter provided should give you some idea of size for the small plants. However, remember that every cultivar is of a different size. For example, a "young" Bohemian Garnet will likely be less than one-half the size of a "young" FTS Maroon Monster.
Having said that...
The "young" size plants are approaching adulthood and will likely flower.
"Young adult" size plants are flowering size.
"Mature" plants are fully grown, large plants.
"Premium" plants are the best plants we offer, the largest in size, the most mature, and in peak physical condition.
More information on sizing, dormancy and other information can be found here:
http://www.flytrapcare.com/store/plant-information