Google it. Wiki has a great amount of info on cups, including all the different brands and their sizing.

You'll find that yes, using either cloth pads or menstrual cups do shorten some (most?) women's bleeding, sometimes by up to two days.

Check out the nappycino cloth mama section - there will be a lot more women there who can speak from thier own experience.

Muppity - given that tampons are associated with TSS whereas cloth pads and menstrual cups are not, yes, what you do use on/for your body does have an impact. Also, tampons can 'dry you out' as they absorb all vaginal secretions, whereas menstrual cups only catch your period so they are a lot more comfortable (not to mention they remove all odours associated with periods, and they're soooo economical and environmentally friendly).