Market Size for Idiot's Guide

Bob Diforio (not sure if I'm spelling it right) has posted submission calls for a lot of Complete Idiot Guidebooks in the past over in Paying Markets. I once submitted a proposal through him for a guide on choosing a counselor. They showed initial interest but then said the market wasn't big enough for that topic. Not sure how your topic compares, but maybe that will give you some idea. I sent the proposal to another publisher who said the same thing but ended up having me write a book on a somewhat similar topic. If you think you can shop it around to other publishers if they say no, then go for it! They have a very, very specific format; Bob sent it to me when I initially touched base with him on my topic.