An 80 lb bag is enough for just about anybody. Maybe 100 or 110 for a HW. You want the bag to swing when you hit it- it is part of how you learn to move around an opponent- so remember to hang it high enough.
The silliest thing I have ever seen was a girl- she was a women's 'world' champion at the time- trying to work on a 240 pound super size bag because the ding-bat that trained here thought it would make her stronger. What it did was get her leaning in and shoving in an effort to move the bag. Killed her footwork and her punching ability.