Determine the size of a parameter from a macro

Is it possible to determine the size of a paramter (possibly a
register) from within a macro in NASM. I have a macro in which I want
to assemble different code depending of the size of the parameter (16-
bit or 32-bit). If it's 16-bit the parameter must be copied to a 32-
bit register that has the first 16-bits cleared and after that the 32-
bit register can be pushed onto stack. If it's already 32-bit it can
be pushed on stack immediately. Simply doing 'o32 push reg' doesn't
solve my problem as the first 16 bits (the most significant bits)
would be undefined...