To assess whether the long harness fits your dog properly: 
Take a picture of the dog from the side, at his level (squat down), ideally in a taut harness - ask the helper as in the photo or leash the dog on and motivate him with a treat or toy discarded in front of him. 

To assess whether the short harness fits your dog properly:
Take a picture of the dog from the side, at his level (squat down). If the dog turns his whole body towards you, ask the helper to guide him to the posture with a treat. 

To assess whether the jacket fits your dog properly:
Before taking pictures and judging the jacket make sure that you have set all the adjustable parts and elastic bands, especially on the neck and at the base of the tail. If the jacket has elastic bands for back legs, put them on. Take the picture of the dog from the side, at his level (squat down). If the dog turns his whole body towards you, ask the helper to guide him to the posture with a treat.