The Dropbox Windows phone app can save as many files as your Windows phone can hold or Dropbox space quota allows.
If you're running out of device space, removing files you've made available offline will free up space on your phone. If you're running out of Dropbox space, you can either delete files from your Dropbox or consider upgrading your account.
Within the app settings you can manually clear the cache, which stores recently used files, by following these steps:
- From your root Dropbox on the Files, Photos, or Offline tab, tap the ellipses (…) button from the app bar and select Settings.
- Tap Cache.
- Tap Clear Cache.