When the Street bikes were first announced, I went from wanting a Street 750 very badly to thinking about the 883 to the 1200 Custom (people tell me I am too big for the 750 - 6'1" about 270 pounds). My Rider's Edge class is this week, so this will give me the opportunity to see how I feel about it.
Welcome aboard first of all.
Assuming you'll be riding a Street 500 on your Rider's Edge class you will have an excellent opportunity to try out the Street. Except for the power difference, which really doesn't matter for most normal riding, there isn't any difference between the Street 500 and Street 750.
There is a huge difference between the Street and the Sportster though as they are completely different mototcycles. It isn't the power or displacement but instead it's the size and geometry of the motorcycles.
I don't believe your physical body size matters one way or the other when it comes to a Street or Sportster but instead it's more of the fact that a Sportster is a beast when compare to the Street. The Sportster has a much higher CG, weighs a lot more, and a "rougher" running engine (i.e. more inherent vibration).
I won't classify one as "better handling" but instead that they're different handling motorcycles. I believe the Street is easier to handle for a "newer" rider than the Sportster personally as it felt more nimble to me when compared to my Sportster (a 2007 XL50) and simply felt easier to ride. I will alsos note that my Street 750 is as fast or faster than my 1200cc XL50 so the extra 450cc doesn't make a difference per se because of the 100 lb difference in the weight of the two motorcycles.
In any case I don't believe your size matters one way or the other. The Street isn't a small motorcycle by any means. It will be your choice based upon what you want in the way of a motorcycle. I would think, given that you haven't ridden in years, that you might find the Street to be more suited to your riding needs of today than the much heavier Sportster but test ride both.