I have a very similar brace for my other hand, in the same size. I not only used the chart here to measure my hand, I used Velpeau's own paper measuring tape that comes with their products (they make an excellent elastic thumb support sleeve/liner that I'd purchased previously.)
The other brand has a similar measurement scale, and the medium size fits like a glove. The Velpeau chart also advises a size medium, but it's noticably larger. There's too much room even after adjusting all the straps, and the plastic is misshapen around the thumb opening (above the adjustable part.) It sits right on my CMC joint and is very painful.
I'm not sure whether to try the small size. This doesn't look like it'll hold up very well, you can see the edges of the glue where the two pieces were joined. The company that makes the brace I like has been out of the size I need for months, or else that would have been what I purchased.
After reading other reviews both here and elsewhere, I'd probably not purchase this again. I like their other products and I've even switched out the Velcro straps on my other brace for theirs, this brace just sadly misses the mark.