I just purchased a pair of Coach heritage stripe rainboots. I got them for $70 on sale, which I think is quite a bargain because they are current season...
Anyways, (I am a guy), I usually wear a 10 and I tried on both a 9 and a 10. I noticed the the actual footsize between the two boots was extremely minimal, however, the leg width (?) was where the difference in size was. I ended up getting the 10's because jeans, ect. fit better in them.
One thing I will say is that the boots smell awful at first because they're rubber. You should also fully inspect your boots before purchasing. The first pair I left with had some type of defect and had a bit of white chipping and gossy smudge.