Plants are typically sold based on the size of the container they come in. For example, a lot of perennials are typically sold in a one gallon (1g) pot. This doesn’t necessarily guarantee the size of the foliage of the plant, but more so the size of the root ball. Though there are some exceptions, a proper one gallon (1g) plant should have rooted through all the soil in the pot in comes in. This can be seen when you remove the plant from the container and all the roots come out with it held together by the roots. The bigger the container size, the bigger the root ball, and the faster your plants will grow once in the ground.
Though most plants are sold by the container size larger trees tend to be classified either by height of the tree or size of the caliper at 5 cm above the ground. Heights are straight forward. For example, a five to six foot (5-6 ft) spruce tree will be at least five feet tall and typically less then six feet. Caliper is used mainly with deciduous trees to give an idea of the size of the trunk and therefor the age and size of the tree. A five centimeter (5 cm) maple tree will have a trunk that is five centimeters thick at the base. The larger the caliper the larger and older the tree is.