Online sizing measurements: Use a soft tape measure to measure in a straight line from the top of your shoulder to the top of your opposite hip. This is your approximate size. If you are in between sizes and have narrow shoulders, you should go with a smaller size. If you are in between sizes and are under 5'2", you should go with a smaller size. If you are in between sizes and have broad shoulders or a broad chest, you should go with a larger size. If you are in between sizes and are taller than 6', you should go with a larger size.
There are no weight/size limits for slinglings. I have a friend that hip carries a 35lber in hers.