Hokay, where do I start?
First, new riders. As mentioned in other blogs and forums-what experience do you have?  I ask this because many "new riders" are new to pavement/city street and highway riding, but not new to a motorcycle as they may have ridden a dirt bike in their past.  They are used to having power at their fingertips, so a slightly larger bike than a real "newby" may start with could be okay.  I do agree with all who have written and get training.  I had training from an experience rider (my dad) which is great if you can find someone who is patient enough to get past your hard-headedness,  although it would be better to use a licened company or group, esp. for your insurance rates.
Size of the first bike.  Hmm...There you have to consisder your size - height and weight.  If you feel like a giant on a toadstool, it is too small - start with something around a 500-650cc.  If you find a much older bike in good shape and a good price, a bike like a Honda CB350, 360, 400, or 450 (or other brand in like size) is great.  I had a CB360 that would have a Vulcan 500 playing catch up (and yes, it was a stock bike) and the 500 is no slouch.  Not only that, but because they're cheap, if you wreck 'em, no big loss in your wallet.  Also, when and if you're ready to move up to the bigger bike, you can usually sell them for the same price, if not more, than you paid for it.
(And...your 70's midsize models, that is, 300-750cc Japenese bikes, are great to practice on before you buy that big cruiser.  The first time a rode the 2004 883 Sportster, it reminded me, in rideability, of the 360 I used to own.)
Another item for 1st timers:  I'll repeat what others have said - practice, practice, practice!