I’m planning on ordering a pair of DS Racers as racing flats for sprints and Olympics. I currently train in GT-2100s at a size 11. I plan on racing either barefoot or with a light sock. I’m thinking of ordering size 10 1/2. Any recommendations on how to fit a racing shoe versus plain old training shoe?
(Yes I’ve looked for the DS Racer in “specialty” running stores. Couldn’t find it. Couldn’t even find the DS Trainer. Go figure)
Depends how you’ve sized your training shoes. If you have a proper fitting with 1.5 - 2cm between your longest toe and the end of the shoe, then you’re probably going to want to go with the 10.5. As they do during the day and during training runs, your feet will swell, even at the Olympic distance. But sans socks or with thin socks, you should be fine. Good luck, and remember…be prepared to return them for a different size since you haven’t tried them on before.
Why don’t you just have your speciality running store order a pair for you to try on? They are usually happy to do this, because if you don’t buy them, they can still sell the pair. I recently did this when I got my DS trainers. They didn’t have any in stock, but they gladly ordered a pair in my size, no obligation.
I also train in the 2100s (love 'em). I wear 11 in the 2100s and 11 in the DS-Trainers. I got the DS-Trainers for IM. I tried on the DS-Racers, and liked them a lot. Too light for IM, but very nice. I don’t know if any of this info is helpful to you, but there it is. 
-C