I’m putting this here, because I know I’m gonna forget it :)
Overall size limits -
On an on premises installation, there are no fixed limits on storage. You can configure it as you wish.
For Office 365 (SharePoint Online), the limit is 7GB per user.
Number of items (documents and folders):
You can sync up to 20,000 items in your SkyDrive (personal library).
You can sync up to 5,000 items from other libraries, per library.
In other words, you could sync 10 libraries (for example), with each library having up to 5000 items.
Third, site limits:
No fixed limits. It depends on your computer's resources.
Hooray! and Happy SharePointing!