I purchased these booties in my regular shoes size, and they fit just fine.  I'd hesitate to purchase smaller because my heels might hang over the back of the sole.  I've used them for an exercise my trainer refers to as the spider crawl, which has been too awkward a movement for regular shoes and caused blisters on my bare feet.  I've also used them for kettle bell workouts and just tooling around the office.  I'm more aware of my foot strike and my arches are getting stronger.  At the end of the day my feet feel energized, as though I'd been barefoot.

I recently purchased the round toe bootie too, after my pleasant experience with the split toe - not as stable.

My one complaint is that they don't come with the rubber outdoor sole.  I initially purchased the low split toe with the rubber sole but the heel slides down and off my foot.  The high split toe solves that problem but doesn't come in the outdoor version.  Hopefully they will start making it.