I went through this same process last summer. I'm 47, had done some dirt bike riding as a kid decades ago, and some small bike (250cc) riding in Europe on vacations in the past 10 years. So not much riding overall. I took the MSF class and got my license. Definitely take the class. For reference, I'm 5'11" and 190 lbs.
After debating how big to go for the first bike, I choose a 2007 Kawasaki Vulcan 900 Custom. I knew that if I really took to riding I'd likely outgrow it within 6-12 months, but if not, at least I wasn't making a huge investment, and in the worst case if I laid it down, it wasn't going to cost me a significant amount.
After riding the Vulcan 900 for 1-2 months, I got very comfortable riding and knew I was going to upgrade. After much research, and test riding several bikes in the 1500-1800 cc range, I settled on the Tbird Storm, and decided to wait for the 2012 models to come out before buying. So I got a 2012 Tbird Storm about a month ago, and have already put 600 miles on it. It's a great machine. Though it weights about 150 lbs more than the Vulcan 900 (which is a pretty big starter bike at 600 plus lbs wet), the Storm features a better center of gravity, and definitely handles better and is less awkward in slow turns then the Vulcan 900. So the irony is that the Storm handles great, better than the Vulcan and some other midsize bikes, but on the other hand, the Storm's got a lot of juice, and it's not inexpensive.
Based on my experience (as someone who got very comfortable riding in a fairly short time), I'd recommend the following options, depending on what fits you best:
1) if in your heart you know that you have a strong passion for riding, and if you have confidence in your ability to grow your skills, then you'll likely outgrow a medium sized bike pretty quickly, so go with the Tbird 1600 or Storm, but proceed cautiously and incrementally. Meaning go to a large parking lot time and again and really get acclimated, and then drive on the roads when there's not excessive traffic so you can grow into it slowly.
2) otherwise if you're more uncertain about it (in terms of your passion and confidence in growing your skills), i'd suggest you go for a midsize bike that fits you, and see how it goes. it's not a problem to simply sell the medium size bike and upgrade within a year's time if that makes sense for you.
So what would I do if I had it to do again? I think I'd do it the same, even though I was kicking myself in a short period of time for not having bought a bigger bike initially. But hindsight is 20/20.
Sorry for the lengthy reply, but hopefully sharing my experience is helpful.