It sounds like you're looking at HVMP bar ends.
http://www.hvmp.com/vespa.htm
I haven't heard anybody complain about getting weights that are too heavy. Why not just get the larger ones to be sure that they're adequate ? It would be a shame to buy ones that don't quite do the job.
I put the larger ones on when somebody was selling a used set. Starreem said that they were the best mod she had done. I hadn't had a wobble before installing them, but did it as a preemptive strike in case changing tires in the future introduced changes.
The only problem that I could imagine with having them too large would be a tiny bit of extra width when lane-splitting, but if you're cutting things that fine you're probably asking for trouble anyways.
Hope this helps. cheers.