Skeleton & Root bone
By default Blender exports the armature like a root bone. To remove this armature root bone Uncheck the box "Add root bone" in Addon Preferences. (Done by default with my addon)
If you want add a custom root bone it you can set the desired root bone name. If the name used is "Armature", Unreal Engine will remove the root bone. You can set the root bone scale too. If you modify this you will have to reimport the animation.
Sine v0.2.7 the addon are adapted to the all workflows. You can work with Unreal unit scale: 0.01 with Blender Unit scale: 1.00 or custom unit scale depending on your production.
This is automatically managed by the addon but if you need you can choose the how the rig need be scaled at the export.
Note: To optimize export time and avoid physics problems I recommend using unit scale at 0.01.