I have small hands (not tiny) and I ordered the small fitness gloves. They fit fine.
What do I consider a small hand (since it's all relative ):
I measured my hand to hopefully help you out.
From the bottom of my palm to the tips of my middle finger is 6".
My middle finger is about 2-1/2" long.
If I measure around the largest part of my palm (like I was measuring my hips) it is 7".
I hope that helps some. I can't tell you how much bigger the mediums are over the small, but if your hands are a bit bigger then mine you probably want a medium.
Well, I got my medium gloves. The fit through the palm/back but the fingers are a little tight feeling. I presume they will loosen up. My main "issue" however, is these are NOT gloves designed for weight lifting (I'm not really sure what they would be appropriate for, honestly). The tag says "cardio" gloves. They have absolutely no padding on the palm, so by the end of the workout my hands were starting to hurt.
In the STS video, everyone was wearing weighted gloves, and some looked like they had padded palms. That's not what's for sale on the website, however, so I'm very disappointed that I won't be able to use the micro-load gloves for actual weight lifting (I'm not a fan of blisters or slipping 20 pound dumbbells over my face). Now I will need to go buy additional gloves elsewhere. Needless to say, this is definitely a disappointing purchase, as it is atypical of Cathe products. :-(