When importing an FBX with a unit scale that is different from UE4's unit scale it will import correctly by unchecking Convert Scene Unit and leaving Convert Scene checked.
If you reimport the mesh these settings will get overridden and the mesh will have incorrect scale.
The mesh will ignore its Convert Scene settings and Scale up to be very large
The mesh will keep the same settings used on import
Head over to the existingAnswerHub thread and let us know what's up.