I would like do it something like this:
macro pushx arg
{
if arg eqtype 32bits
push dword arg; Do something considering arg is 32 bits
else if arg eqtype 16bits
push word arg; Do something considering arg is 16 bits
end if
}
¿How can I know the size of operand, event it is a const, address o register?

The macro argument is simply some text. It has no any "type".