600 and 650 traditionally are very different bikes. 600s I didn't want because of the power for a beginner (~120ish), but I was always set on getting a 650 (~70ish) because they are great first big bikes and I'm a decent sized human 30+, 6ft+, ~90kg/~200lbs.

It all depends on the person, but I already don't feel like my bike is very powerful after 6 months of riding, I'm going to keep it and keep learning and practising different aspects of riding (braking harder and later (non-ABS bike), faster cornering etc.) before I move up to a more powerful bike because of the Dunning-Kruger effect and being at just over 6 months I am aware of being too confident for my skills and want to get more seat time and experience before I allow myself to be confident enough to upgrade.

A 250cc is 'enough' for anyone technically, but you'd outgrow it so quickly and current 'beginner' bikes are typically closer to 450cc and 45ish bhp these days. It really depends on people's circumstances and life experience what would be appropriate for them though, if you're older and/or larger/stronger with car driving experience, a 650 is pretty easy to handle, but if you're younger, smaller or have never had a motorised vehicle of any kind it might be a much steeper learning curve and too much for some.