How can I figure out that how much bandwidth is sufficient for my site to function ?

For that You’ll need to gather some information; fairly easy if you already have a site. Most of this information is available from your traffic history. If you don’t have an existing site, provide an optimistic estimate if you intend to heavily promote the site. Then get ready for some math.

Find out the daily averages of: –
· Number of visitors / expected number of visitors
· Page size including the graphics of the page
· Page views / expected pages viewed by each visitor

Then, multiply them as follows:
Visitors x Page size x Page views x 30 days = Monthly Website Transfer

You should also throw in a small margin or error there to take into account email traffic and your own uploads to the server. If you offer downloads, then you should add the following:

Average/Expected downloads x File Size x 30 days = Monthly Download Transfer