Managing your Dropbox space on Windows or Mac isn't a terribly difficult affair - the platforms have tons of 3rd party disk usage analysis utilities and management tools that will happily do all sorts of things to keep your usage in check. Managing Dropbox on the go, though, is a bit harder. Because Dropbox doesn't sync directly to your device, you need an app that can hook directly into your Dropbox account and analyze the contents remotely.
There are, of course, apps out there that already do this, but I have to say: Unclouded is probably the prettiest one you'll find, bar none.
Filled with fun animations and a modern, clean design, Unclouded is an almost bewilderingly good-looking cloud storage management tool that works with Dropbox and Google Drive. Simply connect your accounts, and Unclouded can do most things the respective storage services' public APIs allow.
Unclouded can find duplicate files (I didn't really test the acucracy well), provide previews of images, movies, and audio, and allows you to display your cloud's largest files or folders for easy management and cleanup. It can even show which files take up the most space on your cloud storage by file type, and display lists of files by the last time they were modified (in the last hour, 12 hours, today, this week, etc).
Of course, actually doing much with all this information will cost you - in order to unlock functionality like moving, renaming, deleting, uploading, native sharing, and creating folders, you'll need to pay $1.99 for the full premium license. The standard app supports, basically, looking at and retrieving your stuff - there's not much you can do with it other than download files you've got stored.
One caveat to note is that the Google Drive integration does not include storage used by Google+ Photos, as Google does not allow 3rd party apps to access that data (so that's on Google, not Unclouded). So, if you're hoping to get a complete readout of your Drive storage, just know that part isn't going to be included.
You can get Unclouded on the Play Store now. Again, it's free to install, and the premium version is a $1.99 in-app purchase.