One thing worth considering when deciding on your preferred thickness of rope is your desired workout and comfort level. E.g. If you plan on performing jumping jacks or outward circles, a 35mm to 38mm battle rope may better fit your needs depending on your physique.
Still, beginners should start with a 35mm to 38mm battle rope. A thicker rope will require relatively bigger hands and a tighter grip. You will spend more time concentrating on your work out and less time worrying about losing your rope.
As an added tip, if you are focused on burning fat with battle ropes, it is worth noting that the 35mm to 38mm Battle ropes lend themselves to more heart-pumping workouts. 50mm Battle ropes lend themselves to muscle building workouts.