[Closed] Rockrider bike sizing

I'm trying to advise my parents on what bikes to buy for their gite in the south of France. It's most likely going to be a couple of bikes from Decathlon, probably the Rockrider 3.5 2012 for 400E. However, the sizing isn't clear on the website.

They list a small (for heights 155-165cm), medium (166-175) and a large (176-185). Any ideas on what's the best size for an average size woman (~165cm) and a short-average bloke (~174cm)? I was going to advise them to buy a 15" and a 17" bike before I saw the odd sizing and it's confused me. Would a small and a medium be sensible?