Here's a decent link on fitting the bosal. http://calclassics.net/php/learn/fittingABosal.php
Muppetgirl - My experience is similar to what you've been told. The thicker the bosal, the milder. Makes sense to me, too, since pressure is dispersed over a greater area (think intro to Physics in general science in high school).
There are many sites about selecting the right bosal for a horse. The only place I differ is that I haven't found it necessary to have 7 or so diameters of bosals. I, and others I know, have maybe 3 favorites that we progress through.
It doesn't sound like that is even your goal, though. Would be great if you could borrow a couple and try them before buying. I will add that buying the best you possibly can is worth is. And taking time to shape it to your horse's muzzle is also worth it.