Try on new shoes at the end of the day. Your feet normally swell and become larger after standing and sitting during the day. Shoes should be carefully fitted to your heel as well as your toes. Try on both shoes. Fit new shoes to your largest foot.
Most people have one foot larger than the other.
Walk around in the shoes to make sure they fit well and feel comfortable. Sizes vary among shoe brands and styles. Judge a shoe by how it fits on your foot not by the marked size. When the shoe is on your foot, you should be able to freely wiggle all of your toes. If the shoes feel too tight, don't buy them.