Re: how does one create a proposal for public sculpture?
Most of the time, you only start making something in clay if you are a semi-finalist (make it throught he first round), and they pay you a stipend for it. But if its a project you really want, or a subject that may have other applications (say a firefighter's memorial for example), it can be worth the extra effort to improve your chances (unless its an RFQ, in which case they probably won't even look at it). For first round RFP's, they typically want only 2d images.
Most maquettes need to be easy to ship and display in a conference room setting, so that limits the size. Big enough to convey your idea accurately. Its really up to you to decide when it is worth it or not. Remember, to get these commissions, you are competing with a lot of hungry artists-- you have to be willing to put the time and effort even when there is no immediate pay-off.
Unfortunately, many committees want to see previous work that is essentially the same as what they are asking for (in terms of size, materials, imagery) so that they know evactly what they are going to get and can trust you with the large budget involved. I suppose that is why many public artists do the same thing over and over. The expectation is that you develop a portfolio of commissions, first with smaller private commissions, then slightly larger public, and then so on. And to complicate matters, it is very difficult to find outdoor sculpture Call-for-Artists for less than $100K that are not restricted to local artists or that are well advertised. So it can be very difficult to find the first small commissions that will get you started.
And then of course, your style has to appeal to the specific committees or communities that you want to submit proposals to, which is often difficult to discern.
Suffice it to say, if you want to do public art, you really have to want it and be willing to put in a lot of unpaid, unrecognized effort. Because in the end, it does take luck to find the right person who is looking for what you are offering. But then that is true with selling art in general, isn't it.