The “stop worrying about engine size” is certainly the best advice you can take. A 1200 cc HD will be easier to master in terms of power delivery than a Duke 390, less than a third its size.
The other advice about getting second hand first is sound too, but if you can easily afford to get brand new there is no harm. Remember thought that you will be caught out and you will drop it, most of the time when manoeuvring at a stand still (sloped curbs are a trap), especially if you are short arsed.