Click on your name from the top of any page to open the account menu.
The bar under your email address represents your Dropbox space usage. You can click on Settings for more details, or click the shortcut here.
Amount of space left on a Dropbox account from the account menu
For our advanced users
Dropbox's performance may start to decline when you store above 300,000 files. At that point you may observe some slowness. We're currently working hard optimizing Dropbox to better handle accounts with unusually large numbers of files.