How much space should be left between a PCB's edge and traces and holes
traces (ground, 5v supply, 9v supply, and 5v digital signals)
  surface mounts
  small through holes (capacitors)


Collectively, all these are covered by the minimum copper to board edge clearance parameter.



The value is driven by the precision with which the PCB fab can register the board before it's routed.  Typical value is 0.020".  Narrower clearance is possible, but the fab may charge extra for additional precision.  So, check with the fab, the clearance should be on their "capabilities" page (like this one).


  mounting holes (for screws)


Distance of mounting holes for screws is driven by a mechanical considerations.  My rule of thumb is to keep the distance between the edge of the hole and edge of the board at least as big as the diameter of the hole.  But this can vary greatly depending on how much vibration your assembly will see, and how heavy are the components on the board.
[hint: Separate question.]