According to 6.7.2 [intro.object] paragraph 7,

Unless it is a bit-field (11.4.10 [class.bit]), a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size.

Base class objects of zero size is a misleading term, as sizeof such an object is non-zero. Size should not be a property of an object, rather of a type.