Is Accelerator able to control the magnitude of vectors? As we know, vectors have both direction and magnitude, and Accelerator's ability is Vector Change. I always believed that he could control both attributes of vectors, until my friends claimed that he could only change vectors' directions.

So is there any clear cut official statement to this question?
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.