I just came across this helpful chart from Vintage Martini for sizing vintage shoes. Perfect timing, because I was recently talking to a friend about how important sizing is when buying shoes vintage online (and how I learned this the hard way...) Every half size is only a change of 1/8" !!
The chart looks accurate to me - I wear a size 8, and 9 3/4" is the magic number I look for. The other crucial number is the width at the ball of the foot - measure a similar pair of shoes you already own to compare. (i.e. Don't measure sneakers if you're buying heels.)
Also, Vintage Martini is having a sale on their entire inventory, 50% off everything starting tomorrow through the end of the year! They don't have an online shopping cart, so you email them to place an order, but they have excellent and speedy customer service.