I think that with a db over about 50-100mb phpmyadmin will not provide a "complete dump" as it'll hit the php maximum execution time and kill the file...
you really need ssh access to use the items in the backup faq entry, but it's the best way to do a backup.
I believe you my be able to find some php scripts which create a dump in your webspace which isn't too out of the ordinary, assuming you can use system() or exec()... Heck, they may even use something differant
