In a smoke-filled room.
Ok, maybe not truly smoke-filled, but essentially so. I'm not sure on the details since I've never worked on one of those transactions, but I sense that once the amount of money needed is determined, the financial advisor does some modeling based on the number homes and their sizes and the interest rate that can be obtained in the market. Longer maturities allow for lower payments on maturity dates.
If someone could host a 4.25 MB pdf on a server, I have a sample Official Statement from a Seal Beach Mello-Roos bond issue that is really boring, but sort of explains how assessment district bonds work. (There are websites that post copies of these in the public domain, so no worries about privacy issues, or anything like that.). Alternately, I can e-mail it to you - evalseraphim@yahoo.com