Also be aware that inside dimensions are much smaller than outside because of taper of structural plastic.  You can lose several inches of length on the 2 biggest sizes and as well as multiple inches on the width.
Mfgs tend to rate the size (weight of the dog) aggressively on these compared to wire crates as dogs are not supposed to spend a lot of time in them.
Don't recommend these for home use as poor ventilation, visibility, no divider for puppies, no removable pan for accidents and spills.  
Hard to clean.  You have to crawl in or take apart.
Don't recommend for car travel as when not using it it only breaks down to half size while a wire crate will fold to 3-4 thick. Also big ones are heavy and awkward to move.
Great for air travel (though be sure to check your carrier's detailed requirements) but not for homecar use.
If dog is a big chewer be sure to get one that has metal wire window inserts not the simple slots in the plastic.