I'd recommend that you print out the Jessie template from the Xero website, then go up a size and a half.
In the Jessies, I'm a W9 (you can see on the profile picture that my toes are just at the end of the sandals). In the Speed Force, I took W10.5, and I certainly wouldn't want any smaller. I wear them either with no socks, or with tights. If you want to add running socks, add another half size.
* So soft. On the Xero website the majority of the reviews refer to slippers. It's true, the upper is very light and slightly stretchy, and the sole is as thin and flexible as the Z-treks / Jessies.
* Sufficient grip. I went for a run in very deep mud over Christmas, and whilst my running partner was sliding around, I only slipped once.
* Protection. I'm using them for running in the forest, and stinging nettles, brambles and twigs just slide off - no more scratches and stings.
* Very light. It's not a completely barefoot feel, but my joints and back are happy.
* Reasonably smart. I've worn them with tights and a nice dress and received positive comments. They're not dress shoes, but the black ones blend in ok.
* Sweaty feet. I've worn nothing but Xero sandals / gone barefoot for the past few years, even in Winter, and so these shoes feel very, very warm. I can feel my feet sweating inside them, and the shoes are probably going to smell baaaaad quite soon.
* Blister. At one point, I had a small blister on the end of my big toe. Probably due to running through the mud I mentioned before, downhill with soaking wet feet. Having said that, I got blisters sometimes under the sandal straps in similar conditions, so it's not really surprising.
I said in a post on here that I was on the look out for something:
I've found them* as flexible and thin-soled as my z-treks / jessies
* long enough (I'm a tall lass at 6'1" / 185cm). To make matters worse, since I've been wearing Xeros after years of cramming into high heels, my feet have lengthened to around 11.5 US / 42 European / 9.5 UK, which makes it almost impossible to get ladies shoes.
* narrow enough (the men's Xero's look like flippers on me)
* smart enough to wear with a suit and tights for work when I have to dress up