I tend to somewhat disagree when it comes to road machines.
The best place to learn is on a dirt bike in a big open field or country roads while wearing appropriate "..Oweee..." equipment like MX boots, gloves, helmet, jacket, etc.
When it comes to a road bike, you need enough 'oooomph' to stop and or get out of the way (..upright..) when carrying a passenger - which most people do. Anything under 500cc is marginal, IMHO.
I had a Yammer-n'-stammer 250 Big Bear when I was 135# and in college, which had the power and brakes to run with the 650 and 750 BSR's and Trumpets. It would almost keep up with the Honda 750-4's, but it was real "..peaky.." (..2-cycle, not seen much anymore..) for a beginner.
My first "..real scooter.." was an '80 Kawi Z1 Classic 1000cc Fuelie. That was followed by several GPz1100s. Big and bloody-fast high-performance also equals quite stable and forgiving, if you're not engaged in the insane to start with. Having a good dose of "...Ludicrous Speed..." power tends to help keep you off the throttle until your "...OMG..." comfort level catches up with your checkbook.
I taught a nice young lady from Martin-Marietta to ride her brand shiny new Kawi 750CSR twin without a whole lot of trouble. She did just great after we lowered the suspension an inch front and rear, and had her riding boots "..raised.." a 1/2 inch.
She suffered one case of road rash when she when out with someone else, who wasn't watching out for her. The road rash incident occured when a dumbass buddy took her into a gravel parking lot off of a major street with no warning or previous explanation. She never whined a bit - although I know she was scuffed pretty good in a few places.
It has a lot to do with who takes the time to teach you to ride on the actual road. Bike school only helps with the technicalities.
I used to take Glenda out every weekend in the Denver-to-Evergreen area, put her ahead of me in the right side of the slow lane - and make damn sure the cars stayed away from her. In no time, she was "..puttin'.." around by herself on her own.
Park your Sub at the iHub - Bub; .....the experience might just "...float your boat..." !!!
.......According to the Great Pumpkin, ".....You're in .....iHub....., Charlie Brown....."!!!