while there are many motorcyclists here, this is primarily a non-motorized cycling site.
As for motor...
1. TAKE THE MSF SAFETY COURSE!
2. This is your FIRST bike, NOT your last (hopefully), so there's no reason why a 125 for the street won't suit you until you get used to it. Unless you're talking dirtbike. You can certainly go 250 without worrying, for the street. I started on a 750 and that was way too much.