The district gives you 20 megs of space as the default amount for your SiteBuilder account. Depending on what you do, this is either plenty, or not enough :)
If you suspect problems (eg. you go to post grades, the same way that you always have and some of the students show up and others don't), it's time to check on this.
To check, log into the District Connection and click on the SiteBuilder application. Examine the first page that comes up closely. It should look something like this:

If you are out of space (or close to it), you have two options: