When someone who doesn't have a website wants to predict their potential usage for a new site, is there any kind of formula you can use? Appreciate that this could vary depending upon the type of functionality required and geographical location, but any help would be greatly appreciated.
Personally, I don't think there's any formula, as it all depends on your marketing and its effectiveness. You can have a website, not tell a soul about it, and have no usage. You can spend thousands on advertising and get only a few hundred visitors. You can spend $100 on advertising and the subsequent word of mouth leads to tens of thousands of visitors (obviously your site must be providing something unique and of great quality for this to happen).
You start a search engine based on some unique approach. It may flop (despite great search results relevancy) or it may challenge Google. You just never know. There is no formula. Marketing is not an exact science.
A webmaster can check how heavy the site is in terms of whether its a flash or an html website. If its quite a loaded site its obviously going to need more space as compared to a purely html site.
You may check this same issue with a couple of webhosts which you think can offer you good deals and cross check them by just login in on to their live sales chat and they will be able to guide you in a right direction hopefully.