Wearing Peep Toe Wedges, Not The Right Size?

I just bought these peep toe wedges yesterday, and they look and feel fine to me. However, there is a slight 2/3 of an inch at the back of my foot. My family says I should've gone to an 8 1/2. {I'm a nine usually.} Is this true? Should I have gone down half a size?

Peep Toe Ankle Strap Wedge Heel Sandals Shoes US 5-8.5