Since the 1.9 update, this mod has been included in the stock game so that it can be loaded on mobile as well as PC. To use it, you should just be able to open the Mods section on the home screen, and enable it.
This mod allows for fine-tuning position and rotation of parts in the editor.
Includes ability to scale(resize) parts.
Now with experimental multi-part mode
Tab for multi-select, then X for multi-edit (multi-select and multi-edit are separate modes for simplicity), or use the GUI button to cycle through them.
Multi-selected parts will all rotate on the axis of the first selected part (unless changed in the mod settings, which require Mod Settings mod).
Scale does not move multi-selected parts when scaling them.
Multi-select selects things, then when things are selected you change to multi-edit to actually adjust them!
Thanks to @Skua for the icon :)