Enable *Clipping* on the mirror modifier:

![enter image description here][1]

To get it to scale completely as it would have before deleting half and mirroring, you must use the 3D cursor as the pivot (<kbd>.</kbd>). The 3D cursor also has to be in the center of the cylinder. The easiest way to get it there is selecting middle vertices and pressing <kbd>Shift</kbd><kbd>S</kbd>*>Cursor to selection*.


![enter image description here][2]


  [1]: https://i.sstatic.net/iMfGr.png
  [2]: https://i.sstatic.net/sSSpy.gif