Bigger is not always better....in hosting that is
I was one person who thought I needed enormous webspace and transfer when I first started out (todays standard packages were huge offers back in 2001), and as a result bounced from host to host. I then took my time, saw what my site used and went a little higher to allow for expansion.
ALWAYS do research into a host. For those offering the 20GB space and 200GB transfer, check their inception date. Most will saw 2011, meaning they just started up. Now figure how will they pay to allocate all their users that amount. Either they are massively overselling, which will catch up to them and force them to shut down, or they are paying ridiculous amounts for their server and will shut down since they do not want to pump that kind of cash in for nothing.
A few pages wont use too much space/transfer unless they are laden with images/other large files. Forums starting up wont use much, unless they are unique and a popular category (or the first of its kind, like a vid game). If you had the site before, look at the usage stats. Allow for initial expansion and always ask the host about upgrade packages.
Personally I would not accept any offer that allows huge space/transfer unless I knew about them first. For me, thats anywhere over 1GB space and 15GB transfer, give or take a little.
Remember though, and something that I cannot express enough, ALWAYS do research on a prospective host. You may take some time doing that, but is far better then bouncing from host to host and taking the time to post here, upload, find the host gone and rinse/repeat.