Yes. That small/medium/large bit is nothing to worry about. For Specialized Rockhoppers: S=15 inches, M=17 inches, L=19 inches, XL=21 inches. I'm pretty sure that Specialized Hardrock sizing is the same. But things do get different with some of other lines. For example, my medium, Specialized Enduro SL works out to a 17.5" frame (or close enough to 17.5").

In the end, the words "seventeen" and "medium" are just labels. Don't get hung up on the labels. Similarly, don't expect a 17 inch Trek to necessarily fit and feel the same as a 17 inch Specialized. Bike sizing is not that exact. It's like with shoes. A size 17 from Trek might feel terrible, whereas a size 17 from Specialized might feel great. And also like shoes, nothing beats getting on a bike and trying it out to see whether you are comfortable on it.

BTW, the brands you mention? Those are all good brands. Don't be afraid to buy from any of them.

Tell DH to definitely try out that 29er bike. He might like the larger wheel size.

Good luck. Have fun looking at all the bikes. Try anything that looks interesting. That's half the fun of it.