Is it possible to define a 3d model/prefab size in unity? - unity3d

Question

Is there a way I can define a 3d model size in unity? Like height = 1, width = 3, depth = 3?
I want the model to take a defined space in unity's scene, no matter how big or small I make the fbx in Blender. So I can't use scale as changing the model size in Blender will break this scaling.
I need it to be a square 3 wide, 3 long and 1 high, not depending on the model's size that is exported from Blender. Is it possible?
The same question but from another angle - how to set model size in unity? There is only the scale setting, but no size setting. This looks weird.
So far I have found a workaround like getting object's rendered bounds and adjusting scaling quotient in a script, but this doesn't seem right to me.

Answer 1

You can use the Mesh.bounds to get the 3D model size without applied scaling.
Then you recalculate the scale according to your needs e.g.
// The desired scales
// x = width
// y = height
// z = depth
var targetScale = new Vector3(3, 1, 3);
var meshFilter = GetComponent<MeshFilter>();
var mesh = meshFilter.mesh;
// This would be equal to the 3D bounds if scale was 1,1,1
var meshBounds = mesh.bounds.size;
// This would make the model have world scale 1,1,1
var invertMeshBounds = new Vector3(1/meshBounds.x, 1/meshBounds.y, 1/meshBounds.z);
// Use this if you want exactly the scale 3,1,3 but maybe stretching the fbx
var finalScale = Vector3.Scale(invertMeshBounds, targetScale);
As I understand you want to keep the correct relative scales of the 3D model but make it fit into the defined targetScale so I would use the smallest of the 3 values as scaling factor
var minFactor = Mathf.Min(finalScale.x, finalScale.y);
minFactor = Mathf.Min(minFactor, finalScale.z);
transform.localScale = Vector3.one * minFactor;