CAD tip # 12152:

Linewidth (thickness) of sketch lines is controlled by settings of a specific color scheme. Launch the Color scheme editor (since version 2019: Application options > Colors > Customize schemes, in older versions see Tip 10596) and in your preferred color scheme change the value Design > Sketch > Thick 3D line from the default value 2 e.g. to 3 or 4. Similarly you can change the value Thin 3D line.

The sketch display will then change from the default:

In older Inventor versions you can also use the Registry settings in the key:

HKCU\Software\Autodesk\Inventor\RegistryVersion2x.0\System\Preferences\ColorSchemes\Schemes\scheme#

e.g. to:

3DThickLine = 4
3DThinLine = 2