6'ft is good, that way you can practice head kicks without kicking the stupid buckles or straps that are more abrasive.
I'm not sure what Kanto is talking about with opponent weights but 100lbs is fine. Just remember for shin conditioning kick as close to the bottom of the bag as you can. Tigerbalm helps when you're starting out.
tip: I mark on my heavy bags with duct tape where the liver, chin, and sciatic nerve would be - assuming the person is average height of 5'9"-10".