I need to find out how much space I'm using on my web server. I'm hosted as well (as in http://me.someone.net).
How can I tell?

Save all the files that are uploaded there into a folder on your comp, right click on it and go to properties, it will tell you the size. I think thats the only way if your hosted rather than own a domain.

sometimes hostees have their own control panel. I know with the Ensim control panel they do- try doing to http://www.[domain].com/cp or http://www.[domain].com/admin (the most likely addresses of the control panel). if there's an option to log in as a user (rather than admin), try that. it should show you how much space you're using including files and databases and everything :)