This is a paid asset, but now you can download Real Size: Measure and Resize with Real World Units for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Real Size: Measure and Resize with Real World Units v1.0.2 (Latest version)
Measure and resize your Mesh Renderers, Skinned Mesh Renderers, and GameObjects using real world units.
Real Size is an editor tool that extends the Transform component inspector for all GameObjects. There’s no need to add extra components to your GameObjects, it just works! Add realistic dimensions to your objects, and keep your world’s scale consistent!
Real Size calculates the bounds of a selected object, including or excluding its children. Bounds are color coded with their respective axis colors, and lengths are labeled conveniently in the scene view. Color and visibility settings are completely customizable.
Works with mesh renderers, skinned mesh renderers, and GameObjects that do not contain meshes
Measure objects using the built-in size units, or define custom size units to measure objects
Measure objects using Local Space or World Space
Easy installation, simply import Real Size and start measuring and resizing instantly!
Full source code included
Tested with Unity versions:
Prior to Unity 2020, baking the mesh of a Skinned Mesh Renderer does not use the transform’s scale. Therefore, Skinned Mesh Renderer bound measurements will only be accurate when their scale is set to 1.
Due to the way Skinned Mesh Renderers bake their mesh, Real Size is not fully compatible with Skinned Mesh Renderers for the following Unity versions:
2019.4 LTS (Partial Skinned Mesh Renderer support)
2018.4 (Partial Skinned Mesh Renderer support)