how to reduce ground symbol size?

[Daxxin]

actually working with very dense schematic and power symbols too big.

[ajb]

AFAIK You can't.  FWIW, I usually use the bar symbol for ground, and the arrows for voltage rails as these are the most compact. 

But if your schematics are so dense that the default symbols are that problematic, then it's probably too dense to be legible, in which case you should go up a sheet size or break the sheet down into more than one subsheet.