Various frame size calculators on the internet spat out 19" for the size of a hardtail mountainbike frame that suits me. The local bike shop insisted 19" is way too small; he only had 21" sized models. I've never ridden a Mountain Bike in my life and I'm only used to handlebar-saddle drops of up to 20cm and ultra long stems. I've test ridden one and due to my lack of judgement I can only describe it as hopping onto a Harley Davidson Chopper, except that the steering was incredibly sensitive and prone to speed wobbles.
According to my research the difference between 19" and 21" is a ca. 1.5cm gain in toptube length and wheelbase. So after correcting with a shorter steam the net difference is a more volatile steering and a longer wheelbase.
I like a stable road bike on a downhill so I am an enthusiastic endorser of smaller frames coupled with very long stems on road bikes. How does a more volatile steering affect a mountain bike on a trail? Since the handlebar is almost twice as wide as a roadbikes, is speed wobble an actual threat for mountain bikers?