How much space/bandwidth is needed?

I was looking at the technical requirements for Dolphin and realized there are no recommendations for how much disk space and bandwidth is needed.  I know this will vary depending on how much activity a site receives. But lets assume your site is a success and is moderately active.

When shopping for web hosting... to avoid falling short or wasting money on overkill, how much disk space and bandwidth should someone consider for a small site  with 1-500 members... medium site with 500-1000 members.... and a Large site with 1000 + members?

Go with these, http://certifiedhosting.com/unlimited-web-hosting/

They are the best and you don't have to worry about a thing. You can add protection to your plan for $5 and they will never suspend your site for over usage ect. They will get there developers to work on your site to optimize and  solve security issues on your site.

@ Bambie

Thanks but, I'm not looking for hosting.  I'm looking for an answer to the question.

It depends on numbers of visitors etc..

Many visitors, traffic on your site = many band pass

A lot of content, photos, videos, modules =  much disk space

This question is very vague it depends the usefulness of your site