Altium has a lot of different rules which stop you placing components on top of each other. All of them are defined under 'Design > Rules...' mostly the conflicts which will come up due to components being too close are silkscreen-silkscreen clearance or silkscreen-pad/net clearance etc etc.
You can change the size of all designators in one hit as per the following,
Right click on one designator, and select find similar objects, a dialogue asking for which parameters must be matched appears, select that all selected components must have string_type=designator and select same, click okay, all designators are now selected.
Using the PCB Inspector you can now change the height and text width of all designators in one hit, you can not change all designators by right clicking on one designator whilst others are selected, it just doesn't work.