How big would a solar panel have to be to bring a 1' x 2' heating element to a sufficient temperature to......

Question

...instantly convert liquid water into steam in an enclosed chamber?

Answer 1

Depends on how much water you have and how "instantly" you want it to be.  Even the hottest stove takes a finite amount of time to evaporate water.

Do the math yourself.  4.183 J/gm-ºC.  A Joule is a Watt*second.  To raise 1 gm of water 80ºC from 20ºC to get it to the boiling point, plus 2270 J/gm to evaporate, so a minimum of 2604J/gm.