…isn’t really about engine size, but mostly about configuration.
“No 600s” really means “no 4 cylinder engines.” Sportbike-style 4 cyl engines have peaky power delivery that requires some skill to handle properly…they’re designed for the high-rpm world of racing.
Twin cylinder engines (as usually found on cruisers) have less power per displacement, but it’s applied in a more linear fashion that’s easier for a beginner to handle.
I’m not a cruiser expert (I’m sure you’ll get better info from Elwood who will recommend an 800cc bike!) but I’d say you’d probably be fine with a 500cc bike, but a 650 wouldn’t be out of the question either.