I bought these to replace some Medimex Plogs that were on their last leg. I would say that they run just a smidge smaller than the Medimex.  I wear a size 9 and my foot is at the end of this shoe. However, I think the next size would be too big for me. You can wear these with or without the insert.
I love these kind of vulcanized rubber shoes and have worn them for years. Especially since you can put them in the washer or in the top rack of the dishwasher. I looked on the Calzuro site, and they were the exact same price as Amazon, so I ordered from Amazon. Wear them on my 12hr nursing shifts with no problem.