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?

Not sure if it helps but I am 6ft2 (187 cm) and I have an xl 8.2 Rockrider, 100mm stem.

The medium and small sound like the job, or even possibly two mediums (might be a bit big for your smallest rider).

Can’t they go in and throw a leg over the bikes then take them away with them? Seems easier than guessing.

Also there’s a sizing guide – http://www.decathlon.fr/ecfr/m/zip/37534_1.zip_dir/vtt.html

I bought a large rockrider 5.2 a couple of years back to get back into biking. Great bike for the money. Still in the shed and I’ve just measured it (19″). Would reckon the medium and small would be what you want although trying before you buy is still the best thing to do.

Edit: I’m 5ft 11 by the way

My parents are going to go to the nearest Decathlon in the next week or so to have a look, but they’re not exactly mountain bikers and they are currently cycling Edinburgh co-op rigid bikes (mum has a 17″, dad has an 18″).

The smallest rider is probably going to be my mum at 163cm, although she’s not exactly going to be gnarring it up to the rad (or something like that). I’m probably going to get most use out of them, and I currently ride a 16.5″ Trek hardtail which is just a tad too big for me.

My gut feeling is to go for the small and medium, with the knowledge that it’s easier to make a small bike bigger with stem etc, than make a big bike smaller!