The Falcon 9 is nothing compared to the size of a Saturn V rocket, but about the same height as a shuttle (with the boosters attached). From Orlando you probably won't be able to see the rocket itself as it climbs but you almost certainly will be able to see the fire plume behind it if it's a reasonably clear day. I've watched some shuttle launches within WDW before, but it's not nearly as thrilling as watching it from the 528 causeway on either side of Merritt Island, if there's any possible way you could drive over there.
In MK, you'd look, appropriately enough, toward Space Mountain. In EPCOT, you'd look toward China if in WS, or toward, again appropriately, Mission: Space if in FW.