[math-fun] propositional logic tautologies

I'm a little rusty on my propositional logic, so I can't remember (if I ever knew) the answer to the following questions: 1. Is there an easy way to enumerate/generate (all and only) *tautologies* in more-or-less increasing complexity fashion? By complexity, I mean more-or-less increasing fashion wrt the number of distinct variables and the size of the expression. 2. What % of prop logic expressions (of a particular complexity) are tautologies?