Guess I'll throw my two cents in.
As others have stated, there just is no right answer. We all think that a good cruiser needs to be a minimum of 900 - and that's considered small these days. And for sportbikes, 600's are considered small and for relatively new riders.
But there was once a time.... oh so long ago:
A 750 was a good size for a cruiser, and considered optimal for doing it all. And the biggest cruiser you could get (other than an HD) would have been about 1100cc, and considered a powerhouse motor. 1000cc sportbikes were considered insane! And they were big heavy sleds, more appropriate for blasting across long stretches of highway than turning laps at the racetrack.
At one point, a CB-550 was a true middleweight, and considered more than enough for trekking cross-country, blasting the twisties, or just pulling daily commuting duty. But not anymore. Now, we all think we need motors that would have powered a compact car 20 years ago, or the bike just won't cut it. (and I'm just as guilty as the rest)
OK, got that off my chest.... :lol:
For todays bikes:
As V-twin cruisers go, I firmly believe that between 1200-1500 is optimal for a do-it-all bike. On the highway, you can pass traffic by just rolling on the throttle, even two-up with luggage. No downshifting and ultra-high RPMs like a smaller bike. And around town, they are still relatively light and maneuverable, to squeeze through traffic without wobbling around in stop and go traffic, like you get with the really big cruisers.
They won't be as lithe as the 900 around town. And out on the highway, they may not lumber down the road as easily as a V2k. But they are a pretty good compromise between the two.
But it really all depends on what suits you the best. At one point, that 900 would have been considered a nice big bike. We've just been riding that upward sliding scale for years, where what is considered a 'good' size gets bigger and bigger with every model-year.