Sounds like you are puting thought into your decision…at 6’4 your choices for bikes will be limited…cruisers are a good choice for the “lanky”, and a 250cc cruiser will move you pretty decently upto 50mph…then progress will slow down…if you are sticking in town for your commute, then this would be the recommended size of bike since they are lighter and easier to move around at a walking pace (which is the speed that usually catches people out)…if you are doing hiway commuting you may want to go to something with a little more oumph to it…upto a 650cc cruiser maybe…you may also want to check out the dual sport bikes like the Suzuki DR-Z 400 s…they have decent leg room, and enough power to hit 95mph…but if a cruiser is your preference, you shouldn’t have a shortage of choices…the only bikes that will be hard to find a fit may be the sport bikes, since their natural riding position is a little cramped for the long of leg…
Anyway, let us know what type of riding you are planning, or need to do so that we can help you out a little better…
By the way, the beginner rider courses are great…they usually give you a chance to try out a cruiser or a dual sport (most schools have both of them), some may even have a sport bike to try…they will also teach you the correct way of riding instead of learning bad habbits on your own and trying to break them later…
Since it is winter, I would hit the bike shops and start sitting on all the bikes that catch your eye just to see how they fit…have fun with it…
Later.