Web hosting is an extremely competitive field and most web hosts will offer some rather large amounts of space in an effort to sell you on their services. But do you really need all that disk space? Is it really more important to puchase 20 GB of space than it is to purchase 10 GB?
Disk Space And Bandwidth
Disk Space is the amount of space you're given to store your files. The common misperception is that you need all the space being offered and so should always go with the host that offers the most.
A small static website typical for many small businesses really doesn't need a lot of space. Let's assume your site is 100 pages and that each page has a file size of 100KB. Further let's assume each of your pages has another 100 KB of images for a total of 200 KB per page.
100 pages at 200 KB each is approximately 20 MB of space. So does it really matter much if you purchase a hosting plan that comes with 5 GB space or 10 GB space? You could fill 5 GB with about 250 copies of each of your files.
It's still a good idea to get more space than you currently need, but having 500 times as much space as you need isn't a much better deal than having 250 times the space you need.
Bandwidth or disk transfer can be thought of as the amount of disk space you can trasfer across the web. Using the above 200 MB site as an example, if one person accesses your site over the course of a month and views every page then you've used 20 MB of disk trasfer that month. If 10 people accessed each of your pages you've transferred 200 MB across the web during the month.
More bandwidth is usually a good idea, but how much you need depends on how many visitors you get to your site and how many pages they attempt to view. Just as with disk space there's a good chance you won't need all the Bandwidth a web host offers.
You'd sooner have more of both disk space and bandwidth, but do keep in mind that most hosting plans will likely offer more than enough. A good idea is to determine as best you can what your needs will be. If you estimate you will be needing 10 GB of space two years from now then make sure the plan you sign up for has at least 10 GB, but don't jump to another host simply because they offer 30 GB.
Real World Examples
I'll use my own site as a real world example. At present I have close to 300 pages on my site. Not overly large, by any means, but probably within range of many small business sites. In addtion to html related files and images I have several databases installed to run among other things my blog, TheVanBlog, and client management software to help run my business.
Even with all those pages and databases my site currently consumes about 70 MB of disk space. Bandwidth, while consistently growing as traffic to the site grows, was still just under 1.5 GB for last month. Admittedly I know how to develop pages to keep their file sizes smal and most of the site is text, with very few images. Still it should provide an indication that the multi-gigabyte offerings with many hosting packages aren't always necessary.
When You May Need More Space
It is possible you're space requirements will be higher. Video and audio files will take up a lot of space and if your site is about either then you will probably want to purchase more space.
You may decide to run a forum on your site. If you do and if you build a stong commuity you may find your database and your disk space growing pretty fast. Those forum posts are generally text only and not needing a lot of space, but space may be more of a consideration with any fast growing site.
Most web hosts will let you upgrade your plan or purchase more space if your needs change at a later date so even if you do find yourself needing more space it shouldn't be an issue. Just make sure to keep an eye on how much pace you are using and if necessary purchase more when the time comes.
Overselling, What Is It And Should You Be Concerned
Many web hosts practice something called overselling. The idea behind overselling is to sell a little more space than the host can really offer, because they know most people won't ever use all they purchase.
Back to the example above let's say you chose a 5 GB hosting plan to host your 200 MB website. Your site is using less than 5% of the space your currently paying for. That leaves a lot of empty space on the server. Again webhosting is a very competitive field and knowing that you'll likely never us all the space with your plan many web hosts will sell that space as part of another hosting account.
In the end they may sell 150 GB of space on a 100 GB server. Obviously there would be trouble if every account decided to make use of all the space in their plans. But reality tells us this probably won't happen and even if it did your host would simply move some sites to another server before it became an issue.
Overselling may seem rather dishonest and unethical, but unless abused there's really nothing wrong with it. While it is possible that everyone would max out their disk space, it's very unlikely. Also servers can be set up easily to send a warning when they are getting full. Your host might have a threshhold set and once a server reaches 80% of capacity some of the accounts are moved to another server to assure that there is always enough space.
If abused though, overselling can be risky. While you and every other account on a server may only be using 5% the space alloted, it would be very risky to sell the space on that server 20 times over. However it's not uncommon to see server space sold five times over.
If you've ever wondered how some hosts can offer so much more than any other at much lower rates it's probably because they are overselling their server capacity more than they should. Their offer of unlimited space for $1.99 may sound great, but you really aren't getting as a good a deal as you might think. That oversold server may not have your space when you need it and may run slower because it's being filled to greater capacity.
And while it may be tempting to purchase the hosting plan that offers the most space keep in mind you may only need a small amount of that space.