I agree with what Klaviator wrote. A 150 is about perfect for city streets. A 250 is equally perfect if you ride on busy freeways. On regular, light traffic freeways a 150 will work fine, unless you are well over 200 pounds. Keep the tires aired up to 28 or so, maybe a pound or 2 more in the back. There's a temptation to look for a bigger machine, but they are not as nimble and responsive as a 150. That ability to dart and dash is hard to find in a bigger machine.
Just remember, NO ONE SEES YOU! You're invisible now. Don't count on anyone doing anything logical. In a little fender bender they get annoyed, you get dead.
Head up, eyes open, and off you go.