Always check the size charts before you buy any running shoes, even if it's from a brand you've used before. Some brands are infamous for their sizing methods; for example, New Balance shoes tend to be half a size under, so if you wear a size 10 normally, you might want to get a size 10.5 New Balance.
Racing shoes are usually tighter than trainers. The best women's running shoes may also be narrower and lighter than unisex or men's options. If you have wide feet, see if they have wide options available – these tend to have a roomier toe box.