I'm 5'9" and feel neither tall nor short in Japan, maybe a little on the tall side. Then again I never felt short in the U.S. either, but maybe that's because I didn't really do online dating. 5'11"-6'0" is more than enough and you'll know it to be true when people start calling you tall when they first meet you.
Japan has a fuck ton of white tourists but relatively few white expats, and as I said in another thread, tourists are often excluded from the dating pool because they don't speak the language or understand the little social customs.
Some urban areas like Shibuya in Tokyo have way more white foreigners. Other cities like Osaka have few. There's advantages and disadvantages to both and it doesn't really matter. The advantage would be that areas with more white foreigners attract women who are particularly interested in them, the disadvantage being that you're "competing" with more white foreigners. It all evens out. And no, not all women interested in white foreigners are highly internationalized, in fact many I've met had never even sat down and spoken with a foreigner 1-on-1 before.
Virgin 16-18 year old gf? It happens. A lot is down to luck. I slept with a virgin 18 year old but it was a matter of just accidentally catching her on her first night out in the city for a friend's birthday party. 16-17 is a tough sell because they are very unlikely to be out meeting people. The only 16-17 year olds I've met have been real creatures of the night, like abandoned by their parents and hanging out with other disenfranchised kids (you see this a lot in Kabukicho). 18 and especially 19-20 year olds are all over the place.