Generally, the bigger the number, the bigger the price, the bigger the plant.
Plants with a number sign (#) followed by a number are roughly equivalent to gallon pot sizes (i.e. #2 = 2 gallon, #3 = 3 gallon and so on). These plants are sold in containers.
Plants with a height (i.e. 5-6') indicate an estimate of the current height of the plant.
Plants measured in inches (i.e. 2.5") indicate the caliper (trunk width) of the plant.
Plants measured by caliper (inches) are generally larger than plants measured in height (feet).
Plants with B&B in the size are sold as balled and burlap (not in a container).
For more information, please see this video that will show you examples and tips on selecting trees and shrubs.
Dianthus
Festuca
Gaura