I voted for "A lot bigger than Mario." I guess the best way I can verbalize my idea of his ideal height would be to say that if Mario was a normal sized human, I'd want Bowser to be at least a foot and a half taller, maybe two, depending on how slouched over he is. Enough to really let him loom over Mario. I am okay, however, with letting Bowser use a transformation spell on himself for the sake of boss battles.