A while ago i injured a toe and have since then struggled when the water is cold. I am looking to get a pair of the O'neill ninja booties 3mm.

My question is, do i get my shoe size in the booties or a size smaller? I am worried they may stretch hence the thinking behind a size smaller (I haven't worn booties before).
If anyone could shed some light on the stretching of booties or if anyone has had experience wearing the booties i am looking at please let me know.
Thanks!