Jasonzilla got a few thumbs down but he has an excellent answer.
If you can practice discipline and restraint when riding, you may be fine on a regular sport bike. It's not all fun and games on a sport bike. Even if you ride perfect you need to know how to react when other cars and bikes and trucks don't drive so perfect. That comes from building new instincts and reflexes, and driving habits. While I definitely recommend the MSF class, the class doesn't usually teach all you need to know. Many, many riders go down every summer due to overconfidence gained by taking the class.
When I ride, I act like I’m invisible to other vehicles. On the highway, I assume that every car I pass or that passes me, doesn't see me and is looking to come into my lane. I'm ready for any move that car makes half a second before he makes it.
You have to learn aerodynamics. If you get to close behind a rig, the wind can trap you there. When you pass along side of a rig the wind will pull you closer to the truck at one point and push you away at another. You have to learn all this and have counter balance and movements programmed into your body reflexes. There is so much more to it. You need lots of practice.
The first time I ever touched a motorcycle, it was a 1997 Ninja ZX-6. I rode it around a parking lot for 5 minutes to find out how to shift, then rode 100 miles home on the interstate. I’ve been riding ever since with a clean riding history. (Thanks to God) So it is possible to start on something other than a 250. Not everybody drops their first bike. You don’t have to drop yours and you probably won’t if you just take it easy.
Some people say that 600CCs is suicide for a beginner rider. That is simply not true. They make 600CC bikes that are suitable for beginners. In fact they make more entry level 600 bikes than they do 600CC race bikes. In no way does a 600CC bike remotely compare to a formula 1 race car. That is an extremist comment you should disregard.
Generally speaking, a 600 is a beginner’s sport bike. If it’s under 599CCs, it’s not really considered a true sport bike. The internet is the only place I’ve heard of anybody starting on anything other than 600 beside people who grew up riding dirt bikes. The Ninja 250R is a scooter with cute plastics. I’ve been to many, many track days, bike nights, meet-ups, and general bike cruises, and still haven’t seen one or even witnessed a conversation about them. They get no recognition in the sport bike community that actually rides. I don't hate the Ninja 250R but it isn't the absolute universal best choice for a first bike the way others would suggest. IMHO, the Yamaha FZ6R is the best entry-level sport bike. The newer Suzuki GS500F is a nice bike. I usually recommend this bike to the people who have been scared about getting a 600. It's got the power to scoot you around town and push you along the highway if you so choose. It doesn't look too bad either.
People recommend smaller CC bikes for beginners because most newbies are into it for the speed and flashiness of it and often underestimate the seriousness of riding safe. The temptation to tap into that power is usually too great to overcome. If you know you can and will respect the bike and get at least a thousand miles of practice before hitting the highway, and before group rides, then maybe you can start on a 600cc bike. Otherwise, help yourself out and start small.
It is also wisely suggested that your first bike be a used one. You may not drop it but the clutch, brakes, and tires take a beating in the learning stages. You don’t want to go tearing up a new bike.
Toned down sportbikes:
1. YZF600R (not the R6)
2. ZZR600 (not the ZX-6)
4. Suzuki GS500F / GSX650
5. GSX600 Katana (laughing stock of sportbikes) A.K.A. Can-O-Tuna
6. Daytona Triumph 600
1. GSX/R 600
2. YZF R6
4. CBR 600RR