just wanted to put my 2 cents in here. I am 5'8" and 155 lbs. I started of on a 98 GSXR 750, I loved it, got a lot of hate from other people b/c I was a little guy on a big bike, took it to the strip every now and then and out ran a lot of the guys there. had it 3 years then it got hijacked. now I have a 2004 CBR 1000RR I have striped it down so it looks like a 600RR now but I still get LOTS of hate b/c I am now a little guy on a even bigger bike that most people think is a 600 till I smoke them! (HAHA). I am on it about 95% of the time I am going somewhere, I love it, long trips, short trips, it doesn't matter, I just love to ride. I break a few rules every now and then, just like most of the rest of us. wouldn't trade it for anything else though. thing is that I am spoiled now, I am used to that TQ that's there when ever I want it. my bike is comfortable enough to ride all the time, and I feel comfortable riding it. here is my point, ride a 400 or a 1300 it doesn't matter, just ride what you are comfortable on, its more about the rider in a lot of cases than the bike. There are a lot of 600's out there that will hang with me just fins b/c the rider knows his or her bike and isn't afraid of using it.
just ride to your skill level and get a bike that will let you improve but not one that will scare you to the point that you wont improve.
"Ride hard, ride fun, but keep the shine side up"