Is Accelerator able to control the magnitude of vectors?
Some proof that he can control the magnitude of vectors:


He can propel himself by changing the vectors around his feet. Seeing as it initially has no speed, and suddenly it has, it means that the direction and magnitude was changed.
When he was first challenged by Touma, he kicked a stone as a warning shot, the stone flew so quickly, it made a nearby tower explode. Again, since the stone was at 0 speed, and went to supersonic speed (because that's what it takes to make an explosion like that), the magnitude was changed.


I'm sure that there could be more examples which I can't remember off the top of my head.