I tried the input below with the following command lines on Windows XP:

dot.exe -Tpng -otest-png -s15 test.dot
dot.exe -Tpng -otest-png -s30 test.dot
dot.exe -Tpng -otest-png -s72 test.dot
dot.exe -Tpng -otest-png -s96 test.dot
dot.exe -Tpng -otest-png -s300 test.dot

That had no effect at all on the final result.

When I added "graph [ dpi = 15 ]" to the input, I got some effect.

The same happened with -Gdpi=15.
Input:

digraph "g" {
	//graph [ dpi = 15 ];
	A [ style = bold ];
	A -> B;
}