Hi cubicapple,
congratulations, much fun and always a safe ride to your daughter - good choice 
Whom do you trust more; the guy at the shop or Mr. Mark Sanders, the inventor of our bikes?
Below…that’s him…does that tell you anything? 
From here: Strida for someone tall
I’m 1,89m, my saddle height on 16" wheels is the highest possible, with 18" wheels one step below.
Just two things should be considered:
- Use the front brake carefully.
- Long legs will need a bent steering, too.
(Knees may interfere with the grips while riding curves…
)