Before you choose a hosting plan, there are many things to consider. Two of the most important are the Web Server Space and the Data Transfer Allowance (also called bandwidth) that you will need. Web hosts will usually try to lure you with either a large amount of Web Space or monthly Data Transfer Allowance. Though the best case scenario would be to have plenty of both, most hosts tend to offer more of one and less of the other, so you will have to find the right balance.

To decide how much Web Space and Data Transfer you need, you must first decide what kind of site you will have. Small business sites generally fall into one of three categories:

One-product website (mini-site): Usually a very small site with two or three pages; its focus is to sell just one product, or to collect visitors' email addresses to try to sell them at a later date. This kind of site is usually just a long sales letter plus an order page and a contact page.
Company brochure site: Usually around 10 or so pages describing what the company does, its products or services, its location, the markets it targets, an about us page, a contact page, etc. It is basically an online brochure of the company.
Theme-based site: Revolves around a very specific topic or market niche (for example: work-at-home moms), and is updated frequently with new articles, tutorials and resources. These sites can also sell products from affiliate companies, or sell their own products.
Each of these sites has different web space and bandwidth requirements.

A one product website doesn't need a lot of web space. However, if the site ranks well with the search engines or has a large number of affiliates promoting it, it will generate a lot of traffic and the data transfer allowance will become very important.
A company brochure site doesn't need a lot of web space or bandwidth. For this kind of site, a basic package offered by a reputable host is usually more than enough.
A theme-based site won't probably need a lot of web space or bandwidth at the beginning. However, if the site is updated frequently with new articles and resources, its web space requirements will increase. Also, as the site gets more traffic and offers more content, the data transfer allowance will also become important. You must plan for the future from day one, and get more web space and data transfer than you seem to need at the beginning.
Here are some expamples of how you can estimate and calculate your web space and data transfer requirements (we will assume that the average size of a web page, including pictures, is 50 Kilobytes).

One-product Site

If your one product site has just three pages, it will just need 150 Kilobytes of web space (a drop in the bucket considering that basic packages nowadays offer upwards of 50 Megabytes of space). However, if it receives 150,000 page views per month it will require approximately 7.5 Gibabytes / month of data transfer.

Company Brochure Site

If your company brochure site has 10 pages, it will only need 500 Kilobytes (aprox. 0.5 Megabytes) of web space. If it has 5000 page views per month, it will need 250 Megabytes (aprox. 0.25 Gigabytes) of data transfer per month.

Theme Based Site

If your theme-based site starts off with 50 web pages of content, it will need 2.5MB of web space. However, if you add ten pages of new content per week, you will need 26 Megabytes more in your first year. After two years, you will need 52 Megabytes, and so forth. As you see, your need for space may add up pretty quickly. If we estimate that your site will have 30000 page views per month, its monthly bandwidth consumption will be 1,500 Megabytes (aproximately 1.5 Gigabytes). This figure will most likely grow as you add more pages.

Your space and data transfer needs will also grow dramatically if you offer large files for download. For example, if you give away a 1 Megabyte PDF Ebook and 1,000 people download it in a given month, you will need 1 Gigabyte of data transfer just for that one particular download (and you still haven't accounted for the data transfer consumed when users access and browse your site!).

Since web hosts will usually charge you fees if you use more than your alloted web space and bandwidth, you must carefully monitor your consumption of both. However, you should pay extra attention to your bandwidth.

The reason is that it is easier to control your web space use (after all, it is you who decide how many files to load up to your host's server). Data transfer, on the other hand, is not as easily controlled. For example, your site may suddenly get a good search engine ranking for a popular search term and receive a traffic boost, which will consume more bandwidth than you had originally planned. Or, unscrupulous webmasters may "steal" bandwidth from you by linking directly to images on your server, instead of saving them in their own web server space.

Therefore, it is wise to plan for the unexpected, by making sure that your host doesn't charge unreasonable fees every time you exceed your web space and data transfer allowance, and that it offers a free and easy way to upgrade to a better hosting package whenever the need arises.