Don't forget the PCB Filter panel, so you can do powerful queries like
IsComponent AND (Name Like 'R*')
to select all the resistors.
Then go to Inspector, click the "Designator" link and change the properties, e.g. size, autoposition.
I don't think there's any way to change the autoposition offset, unfortunately. While you have the Designators selected, you can M,S (Move Selection) to get them closer. Don't forget to switch them to Manual autopositioning, otherwise their positions will reset next time you move the components.
Or save a step by filtering:
IsDesignator AND (Component Like 'R*')

Or filter by rotation (you can't filter by a designator's component angle; you have to select the components first, then follow the link) and position rotated labels, or...

Tim