The measurements of your crate are very important for your pet's safe, comfortable confinement and travel.
This diagram will give you a little help
Length: is from the from the front breast bone to the base of tail plus 2" for medium breeds and 6" for large breeds.
Height: is from the floor to the shoulder plus 3"-6". (For airline travel, it is necessary that the crate be as tall as your pet's head, and he should be able to stand in a natural manner as well as turn around and lie down.)
For housetraining you might choose one that will fit your pet as an adult so that it provides life-long service. To estimate his adult size check the breed standard. (Most breed standards, however, measure your pet at the shoulder.)
Width: is a couple of inches less than height at the shoulder.
Remember, Wire crates can never be used for airline travel.