Importing FBX files in Unity with the correct scale, rotation, and axis is fraught.

FBX is in cm by default, but can support other units. Other unit scales are achieved in FBX by by setting GlobalSettings→Properties70→UnitScaleFactor property in the file. For example, a 1 meter cube in 'meters' should be stored in an fbx file like:

GlobalSettings:
  Properties70:
    UnitScaleFactor = (100.0)   # The 'unit' is 100 * 1cm 
CubeObject:
  Lcl Scaling = (1,1,1)

This file will show up in Unity's importer as '1m (File)'.

In order to achieve the above scaling (scale of 1 on objects, UnitScaleFactor=100), in the export settings set one of the following for Transform→Apply Scaling:

• FBX Units Scale
• FBX All

In the FBX file format, global axis alignment is stored in the GlobalSettings:

• UpAxis: 0,0,1 (+Z)
• FrontAxis: 0,1,0 (+Y)
• CoordAxis: -1,0,0 (-X)
• OriginalUpAxis: ??? ^{[research needed]}

Unity uses these properties to determine the coordinate system for the file. On import, the asset will be rotated to convert it from the file-coordinate system, to the Unity coordinate system. As of 2020.1, when “Bake Axis Conversion” is checked, these rotations will be baked into the meshes, rather than applied in the transforms.

Instructions for Blender ~2.92

Instructions for Blender ~2.83