Hi Pluto,
Right now the only real way to scale something uniformly across all 3 axes is by using a multiplier on all three dimensions in their editboxes. If you want to double the size of something just add "*2" after each dimension and it will calculate double that value. If you want to manually scale something you can preserve the aspect ratio for the XY axes by holding CTRL+SHIFT while dragging the handles in the view. If you want to scale the Z by an equal amount you'll need to calculate the scale factor that resulted by dividing the original X size of the layer by its manually scaled size which will give you a value you can multiply the Z size by to scale it up by the same amount that the XY axes were manually scaled.
It's been on the todo list for a while to add some means of locking the aspect ratio for layer dimensions so that if any of them change the others will update to retain the relationship between all three dimensions, but we haven't quite figured out what that should look like in the UI just yet. With all of the other buttons all over the origin/size property edit boxes we're concerned about things becoming cluttered on there :P If you have any ideas/suggestions, something like a checkbox somewhere, we're all ears! I'm partial to perhaps having a menu with options in it for locking the aspect ratio of different combinations of axes, i.e. "Lock XY Aspect", "Lock XZ Aspect", "Lock YZ Aspect", "Lock XYZ Aspect", etc...
- Charlie