I have never listened to the "small bike first" rule. If you get a small bike now, chances are in a year or less, you will want something bigger. A good friend told me before I got my first bike (a CBR 954) "get the bike you WANT. If that bike is a bigger bike, just remember, there are more than 2 positions to the throttle.
Best advice is find the bike that fits you physically and financially. Take some time to learn the bike in low traffic areas, and you will be golden! Good luck and have fun!
There is a bit of a difference betweeen sportbikes and cruisers though.
The biggest concerns with a sportbike, is clutch control (too high of an RPM and too quick of a clutch let off and you'll be standing in the same spot but the bike will be half a mile away!), and of course having the wherwithall to know your limits and not use the power beyond what you can handle (and being realistic ABOUT what you can handle).
With a cruiser, some of those same concerns aren't quite as much, save for big power cruisers like the V2K or some of the crazy V-4 machines. However, WEIGHT is a big concern with a cruiser. New riders on cruisers will often find themselves stopped at an angle or just being 'awkward' with the bike as they get used to it. An experienced rider will avoid those same situations, a new rider might not. They also may not be used to the weight. New riders on big cruisers (and I'm not saying people don't do it, I know lots of folks whose first bike was something like an Ultra Classic Electra Glide), are more apt to drop the bike than on a smaller cruiser, damaging that expensive new bike!
It's all purely individual. A larger guy with a lot of leg strength can handle a lot more than, say, my 5 foot tall wife!
You hit the nail on the head when you said 'fits' though, because FIT is the most important thing. Go check out some Harley riders and see what they think about small displacement rules. They'll scratch their head and say "what's that?". SOME of them MAY have started on a 900 or 1200cc sportster, but aside from that Harley only makes big displacement twins (currently 1600cc and up). LOTS of folks start on those. They find one that fits right, and go! But again! LOTS of them drop them in the first couple months of owning them. So the OP just needs to figure out where he is in that. Maybe sit on the 1700 in a dealer and lean it left and right and see how heavy it feels compared to the 900, and determine which is more ideal. I think you'll find that both the 1700 and the 900 feel about the same aside from the weight. I've ridden the 1700 (Classic and Voyager) and both times I really regarded them as a heavy 900, because they felt pretty much the same fit-wise.
That said, the suspension and amenities of the 1700 sure was nice! However, knowing a couple situations when I was learning where I ALMOST dropped the 900, I WOULD have dropped a 1700, so I'm glad I went the 900 route. The 900 is also a bike folks aren't in a hurry to trade in. It's not that the 1700's aren't nice, it's just that the 900 is a lot more bike than you'd think a 900cc machine would be. I'll hang on to mine for a long time I think.