When I first used a pair of weighted gloves, I went with a 1lb pair. I thought, there is no way a pound of weight is going to do anything. I was wrong. Despite the feeling that the weight will not be enough, the toning benefits of just the 1lb gloves are great, in my opinion.
I tend to throw my punches pretty hard (not overextending though) and I believe anything heavier then what I currently use could possibly lead to injury. I guess it depends on the individual.
Hope this helps!