Clr Via C# is a good source for this kind of question.
It's relevant to know what kind of size are you talking about?
The consumption of class variables in memory?
The size of the class itself? (Inlucding memory tables(Methods,meta data) and meta data(used by reflection))
For your direct question:
It depends what you're asking. The physical size of the class will be three pointers because A is enscapulated as a pointer under a C# reference.
If you ask for overall size, it will the size of B + 3 times the size of A.
Some performance counters and matrices state the later while some will give you direct answer for the former.
WinDBG is a great tool to check that kind of sizes.