I really think that decision should be based on your particular child. My daughter was never big on putting things in her mouth and so we would let her play with things that were smaller in size - but even then, when we introduced toys she was always carefully supervised. She's three now, and she's been playing with "regular" LEGOs probably since she was under a year. They're everywhere in our house, so we just took the time to train her not to put them in her mouth and supervised her closely at first until we were sure there was no danger for her. Some kids might not listen well, or may be more curious and that might cause you to be a little more cautious.
I don't see any harm in getting her a small Duplo set, seeing what she does with it, and putting it away on a shelf if you think she can't handle it yet. But always supervise!
Joelle