M.N.
I'm jealous! I was born and raised is Ft. Lauderdale. Generally the retirees that winter in Florida come down around Thanksgiving and stay until Easter. If you plan on staying in the Miami area it shouldn't be too crowded as long as you stick to "younger" areas like South Beach, Coconut Grove, etc. The weather will probably be in the low 80's to upper 70's. HTH.
Enjoy!!