In my day we all started out on 100-200cc bikes, then moved up to 350s or 450s, then got a -big- bike. In those days that was 650.
It's a matter of how much weight you can handle. A 400 sounds big for someone who's been riding a bicycle, but you can handle it. From a 400 to a 600 I don't think you'd notice much difference. There are a lot of good 600cc bikes that are very small and light. And that's a good all-around, general purpose size, maneuverable through cities and enough stamina for long freeway trips.
Go to a dealer and sit on some of the bikes and lean over a few degrees each way and see if it seems like just too much weight.