Creating a right hand in Graphics3D to illustrate the right hand rule

It's easiest to control the position and size of all 3D objects if they are combined in the same Graphics3D. For an STL file, this could be done as follows (I didn't want to look for a hand model, so I chose the seashell model built into ExampleData, since it's also chiral):

Export["g.stl", ExampleData[{"Geometry3D", "Seashell"}]];

g = Import["g.stl"];

Graphics3D[{
  Map[{Apply[RGBColor, #], Arrow[Tube[{{0, 0, 0}, #}]]} &, 
   2 IdentityMatrix[3]],
  EdgeForm[], FaceForm[Orange, Brown], GeometricTransformation[
   g[[1]],
   Composition[
    TranslationTransform[{0, 1, 0}], 
    RotationTransform[Pi/2, {0, 0, 1}, {0, 0, 0}], 
    ScalingTransform[{.5, .5, .5}]
    ]
   ]
  }, Lighting -> "Neutral"
 ]

Here I called the imported STL graphics g. To insert it into the existing Graphics3D containing three axis arrows, I strip away the Head by doing g[[1]] which then contains only the polygon data. Also, I added the basic machinery to move, resize and rotate the seashell in the 3D scene, by wrapping g[[1]] in a GeometricTransformation containing a Composition of the three geometric operations. In particular, the ScalingTransform can be used to adjust the size, before shifting the object to the desired location with the TranslationTransform.