It depends on OS version in windows.

The size of the reference itself will depend on your processor architecture - 4 bytes on 32-bit, 8 bytes on 64-bit.

The reference itself is basically a pointer. 32 bits on a 32 bit OS, 64 bits on a 64 bit OS.

All references are the same size (they are 4 bytes on a 32-bit CLR and 8 bytes on a 64-bit CLR, but within any one CLR all references are the same size)