Eyy, hello, Welcome, and welcome me too, im a Rookie here too.
i can say this, i had 250, i felt its completely Fine. 30hp V2 engine. Riding in high gear, no choice on highways.
if you City ride, 300 is Perfect!
if you more adventure and if High speeds above 100kmh, then id say 650 is more suited for that.
300, Super fun, maximum throttle all the time possible. 650, well thats a big bike...With big bike,Comes Big responsobility bla bla bla.
Try both, and see what you will like!
Totally Fine to get the 300! if we talkin about power and fun.