Use free software like Blender or Meshmixer to scale a polygonal 3D model—both support common formats (OBJ, STL) and have easy scaling tools.
In Blender: Open your model, switch to Object Mode, select the mesh, press "S" to activate scaling, drag the mouse or type a number (e.g., "2" for double size), then press Enter. In Meshmixer: Import the model and use the "Transform" tool to adjust scale numerically.
Save a backup first, and use numerical input (instead of dragging) for precise dimensions—critical for 3D printing or design where exact sizes matter.