<p>Not of the site collection itself, but the individual SPWeb's.</p>

You should take a look at this blog entry by Alexander Meijers :
Size of SPWeb based on its Folders and Files
It provides a clever way of finding the size of an SPWeb or SPFolder by iterating through his content.
```
```
private long GetWebSize(SPWeb web)
{
    long total = 0;

    foreach (SPFolder folder in web.Folders)
    {
        total += GetFolderSize(folder);
    }

    foreach (SPWeb subweb in web.Webs)
    {
        total += GetWebSize(subweb);
        subweb.Dispose();
    }

    return total;
}
```
```