If you're in Europe and passed your A test you're about ready to start learning how to actually handle a small bike.
Get extra instruction, and start doing Moto Gymkhana. Consider getting a cheap small and light bike like a CBF250 or Duke 125, and learn to really master it.
Your instructor probably shrugged when you said you were getting a TransAlp because he knew there was no point in telling you not to. He's seen umpteen people pass their test and then go out and crash right away.
You didn't go for a Boss Hoss? That Rocket must feel tiny.