There is a program called Beyond Compare that will perform a comparison of local and remote folders via FTP, but it works on Windows only. I don't know if there's a similar program available for Mac OS X.
One way to find the number of items in the public_html folder on the remote server is to log into the site's control panel, and see if you can find the number of items listed there.
Alternatively, select Remote Server in the Dreamweaver Files panel, right-click the public_html folder, and select Synchronize from the context menu. In the dialog box that opens, select Entire [Site name] Site for Synchronize, and set Direction to "Get newer files from remote". Then click Preview.
Because you have such a large number of files, it will take Dreamweaver a long time to get all the information. In fact, you might find it just as quick to create a new site definition in a new folder, and attempt to download the site again.
