Re: how does one create a proposal for public sculpture?
From my experience (also with no public commissions after many applications), Brad has given a good run-down of the overall process. From what I have seen, selection groups often have a specific idea in mind even though they can't mention that in the RFQ or RFP. I suspect they may even have a specific sculptor in mind, but need to go through the general process for legal or public-relations reasons.
On the matter of making a specific figure that could be enlarged for the final piece, I’d say that is unrealistic. My indirect experience is that the final piece often evolves during a discussion between the sculptor and the commissioning group, and that a first drawing or even a first maquette (small example) is only a starting point for the final design.
Also, don’t overlook the possibility of working with one or more other artists in submitting proposals. I know this idea doesn’t appeal to many beginners but, again in my experience, I’ve often seen that in successful proposals. The commissioning group may want to “grow” or enlarge the local pool of commissioned sculptors.