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Its hard to say but wordpress its self is about 25MB, if you include a simple theme and ofcause the 5 pages you will be looking at about 40MB per site.

If we are talking about simple HTML web pages, you're talking less than 100 KILOBYTES per page (assuming no graphics). which translates to 10 pages per megabyte or about 10,000 pages per gigabyte of storage. Of course each domain account on the hosting plan will eat some of the storage space, but in purely HTML pages, you're talking about tens of thousands of pages.

If you use graphics, the more graphics that are used, the more space you'll use, but using the same graphic file from the same disk location on multiple pages would have three effects:
a) Only one copy of the image exists, so it takes up disk space once
b) The image coming from the same place and same file name usually doesn't get re-sent to the user every time they change pages, so bandwidth utilization is smaller.
c) Since the image is sent once to the user, but seen on multiple pages, the pages seem to load faster (because the images aren't re-downloaded)

If you want to do something more than simple HTML, then the figures get hard to predict quickly. A 5 page wordpress site like Solado said will probably be in the neighborhood of 30-60 MB depending on your theme.

Then calculating against bandwidth is your next problem. You can have a 40MB site, but the bandwidth usage for someone to view the entire site one time is generally much less than the total disk space usage, because the scripts that run wordpress aren't sent to the user, they 'assemble' the html view the browser receives.

But if you're running 1000 sites at 5 pages each assuming wordpress and an average of 45MB per site you're using 45000 MB or about 45 GB of disk space. Your bandwidth probably isn't a problem unless you have a lot of sites running hundreds of views per day each, but your CPU Utilization could be overwhelming if there is an average of 1-2 visitors per site in the same second, you have at least 1000-2000 requests for the CPU to process your scripts in that same second.

If you have dedicated CPU/RAM/Drive then you are probably still okay if its a decent server, but the variables are too widespread to give you a good rough estimate. When I set up a new site, I usually allocated 1GB HDD and 10GB Bandwidth to the site until I work out exactly how I want it, then I watch its performance (and have alerts set to email me if any limits are approached)

I had a problem with one wordpress setup that was creating massive amounts of data piling up in the WP database, and I had to up the HD Allocation 7 times to keep the site from crashing before the developer figured out the problem. (basically it made the database grow almost exponentially until he fixed the glitch)