There is lots of space and bandwidth talks going on these days with hosting companies trying to surpass each other, customer running after space and bandwidth, seems more of a fashion than any one really knowing what he/she needs in this area, May be this needs to be discussed:-
How much Space you need:
Think of your web hosting account as a sub-directory (or folder) on your hard drive. To determine how much disk space you will need, In Microsoft windows simply open explorer or my computer and click on the folder that contains your web sites files. Create a new folder for your web site if you don't have one and then move all of files you plan to host on the web server into that folder. All you have to do now is right click on your folder to check the size of that folder and now, you know how much disk space you will use on the server. This entire site is around two megs.
How much monthly (bandwidth)Data Transfer do you need:
Try using the following formula to estimate your site's monthly data transfer.
[Average size of your web page(s) + any graphics included within] * [number of visitors you expect each day * number of pages each visitor will view] * [30 days in a month] = Total Monthly Data Transfer Usage.
For example: if we had a site with 30 pages averaging 8 KB each, 50 KB worth of images in each page, and 50 visitors each day who viewed an average of 4 pages, you would have the following formula:[8 KB + 50 KB] * [50 visitors * 4 pages] *  = 348,000 KB So we would be using 348,000 KB, or approximately 340 MB, of bandwidth each month. Well within the limitations of our hosting plans.
It's hard to generalize how much data transfer a site will use without looking at it specifically, but in most cases it is very rare for a personal or small business site to use more than one gigabyte (GB) of data transfer in a month. Starting with a data transfer limit of one gigabyte per month is probably appropriate for most new sites. If your average web page is 20Kb in size. 1 Gig of transfer allows for well over 50,000 hits per month at that size! If your average page size is smaller obviously more hits per month.