Files in excess of 200mb are no problem. The first problematic filesize you will run into is at > 2gb (Apache on DH will not serve any files larger than 2gb, and indeed not even display them in directory listings); this is a limitation of apache though, the filesystem can handle larger files than that. I’ve had files in excess of 5gb on the DH filesystem without problems (and you can even create files several dozen terabytes in size if you use sparse files
As others noted, the upload limit in PHP-based uploaders is a lot lower than that (and for good reason – PHP wants to store the file in memory first before it even gets to the disk; a 200mb file would eat 200mb of memory while uploading – that is just unacceptable). It is, however, no problem to upload large files via SCP, SFTP, FTP, rsync, or fetch them on the shell with wget, ftp, etc.
Dynamic websites are websites that require server side processing beyond “just serving a file”. Anything that is PHP is dynamic, for instance, since the server has to execute the PHP code to dish out content to the viewer. The opposite, a static site, is a site that only uses static files (i.e. simple .html files with links to images, files, etc.). A forum is dynamic, a directory full of game patches is not.
DH can handle either with grace, so long as your dynamic content is not coded shoddily and you do not expect huge numbers of visitors.