Go with the same size they wear in clothing. If they are in between sizes or chonky, size up.
All measurements are taken in inches while the item is laid flat.
Chest measures the circumference of the shirt at the widest part, just under the armpit where the seams meet.
Length measures the front length top of the zipper to the bottom of the shirt (the back will be longer).
Sleeve measures from under the armpit to the wrist.