Electronic symbols standard sizes
From IEC 60617 (preview) :


  The meaning of a symbol is defined by its shape and by its content.
  The size and line thickness do not affect the meaning. The minimum
  size of a symbol shall be such that the rules for line thickness,
  spacing of lines, lettering, etc. can be applied. Within these
  constraints, the symbols intended for installation diagrams and
  network maps may be enlarged or reduced to suit the scale of the plan
  or map.


Refer to page 6 of the referenced document for more info.