Even though riding a dirtbike makes you very familiar with the controls on a street bike... the riding styles are completely different. I am sure you know this already, but putting a foot down at speed for a turn isn't really a viable option on the street.... and neither is dropping your bike (unless you like to spend money on repairs and hospital bills). Unless you get a very light motorcycle, a streetbike is going to feel like lard-assed whale in comparison.

Since you didn't say how much and how long and how often you have ridden a dirtbike, I am goign to assume that you are a novice rider (as most dirtbikers are when it comes to street bikes).

It all really depends on how fast you want to learn to ride, in my opinion. Any joe-blow off the street can figure out how to accelerate and decelerate in a straight line in 10 minutes. After you have the basic operation of the bike down, it is things like cornering, leaning, obstacle voidance, risk mitigation and route/path planning that make you get better.

These things are learned faster on smaller bikes... plain and simple. Popping the clutch during a start or on a downshift on a 250cc or a 500cc will make the bike moan and complain. Doing the same thing on a 100+ hp bike might wheelie it, or dip the front fork and make it throw you. Really, it comes down to weight and hp/torque rather than displacement (even thought the two are related).

A 250cc or 500cc bike with ~40hp will be the fastest to learn on. A 500cc is preferable to a 250cc because a 250cc bike will just BARELY keep up in the slow lane on a highway (forget about passing cars... accident waiting to happen). Some smaller riders might need a 250cc bike due to their size, but don't despair... 250's are perfect for regular street traffic.

The highest amount of hp i would start off with is 80hp. 80hp is just barely enough to get you in trouble/when you make a mistake. However, in endeavoring to stay safe and right side up on the motorcycle, you will definitely encounter a steeper (slower) learning curve than a person on an easier-to-control bike.

I'll write some more tomorrow after i wake up. I'm sleepy!