Is a 300cc to small for a starter bike?

Deciding between a Honda cb300f, ninja 250, or an R3. Is a 300 to small?

Not at all. It can still get up and go.

Started my 55+ yrs of riding/racing on a Honda Super 90. Learned to ride and still alive.

Decide for your self- most 300s can do higher than speed limits in the US.

I gave the boys a 50cc bike to start out with. If you're an oldbloke just starting to ride its too late don't bother you will hurt yourself.

How Heavy are you? 153#? Then, no.

200# + ? Yes. You need a 500 or 650.

It's OK, for off-road.

No 300cc is not too small. Those bikes are beyond what most riders in Europe can start with. I started with a 50cc bike, then a series of 125s and 250s.

Not unless you weigh over 250 lb.

All of those bikes mentioned are more than capable of reaching and sustaining highway speeds, yet are not too powerful as to easily overwhelm a novice rider.

The other nice thing about all of those bikes is that they're in constant demand by new riders. Buy a used one and assuming you don't drop it or trash it, ride it for a year or two and you can easily sell it for close to what you paid for it.

A 300cc bike or smaller is ideal as a starter bike. Gain experience and live to ride another day. You will make mistakes so that's better at low speed because you're more likely to live to ride another day.
A friend of ours passed his test and bought a Yamaha Thunderace 1000cc and within a month he was dead.

It is for me. I don't know what you might be able to ride safely.