Well I think this has pretty much been answered; but I think that the width is just a matter of preference and what looks good on your dog.
I usually use/make 1.5" martingale's for our Lurcher, Toby; 2" collars just look to big on him...the other dogs have 'regular' collars, but I do use martingales on them occasionally too (usually when working Obedience or when we are just starting Agility, after they know the obstacles they run 'naked'), I like to us 3/4" with them when I can find it since they (Scout and Willow) are mcuh smaller than Toby.
Amanda (Am_04 on GT)