Well, first of all, depending on what you deleted, you may well have trashed your install. The thing about Linux is that it tracks all installed files (system and apps) in a database, and it really doesn't like you messing with the tracked files (or the database, for that matter) manually.
To answer your first question, if you want to list all of your files/folders, you could normally use ls --recursive, but the ls command on the N900 is a slimmed-down version, and I don't know if it has that switch.

The real question is, why are you running out of space? Deleting random files is likely to do major damage, and in any case isn't a proper solution. Have you been installing things from extras-devel? Due to the storage layout of the N900, apps have to be packaged properly or else they will fill it up really fast. I suspect some of the apps in extras-devel may not yet be done that way (they weren't all when I last checked).
No offense, but I think you may be messing with things that you don't understand fully, in which case it's usually better to ask before you take such drastic action. We've all done silly things when learning a new platform, but we're happy to help newbies to avoid doing those things themselves if we can. If your phone still works enough to run a backup, do that with the Backup app, and then reflash - that's my advice.