Hey from sunny SoCal

First off, I just want to say that I absolutely love this site!. I don't know really know anyone that rides and have been searching for a good resource for advice and knowledge and I am happy to say this website is just what I have been looking for. I had been interested in riding for a few years (I have always loved the look and the sound of a motorcycle) but recently the rise in gas prices have made them even more attractive for their fuel economy. Anyway I have been researching for a good 2 months what kind of bike I should get and the costs of insurance, etc. I am planning on taking a MSF course as soon as I can get the money together for the rest of my gear (and a bike, of course!). I am almost certain that I will be getting a bike in the 250cc class (I'm leaning towards the Honda Rebel), based mainly on the great information I have gleamed from the users of this site. I feel like that would be ideal for me to start on because I am still a little nervous about riding. However, I am a pretty big guy (5'9", 240) and I am worried that a 250cc won't have the power I would need to take it on the highway as I would use the bike mainly for commuting to and from school/work. I know that 250 is considered by most to be easier to learn on for new riders but that 500 is also doable. Is there a huge power difference between these two engine classes? I'm fairly certain a 500 would be more than enough for what I need, but is it worth the extra effort to learn on a more powerful bike?