To measure the size of big dog clothes, you'll need to take a few measurements of your dog:
Measure the dog's neck circumference by wrapping a tape measure around the base of the neck, where the collar would normally sit.
Measure the dog's chest circumference by wrapping the tape measure around the widest part of the chest, just behind the front legs.
Measure the dog's back length by starting at the base of the neck and measuring along the spine to the base of the tail.
Once you have these measurements, compare them to the size chart provided by the clothing manufacturer. Different manufacturers may have slightly different size charts, so it's important to check each one carefully. Generally, it's better to err on the side of a larger size rather than a smaller one, as it's easier to adjust a slightly larger garment than to try to stretch a garment that's too small.