How can I get value of "scale factor" (FBXImporter -> Meshes -> Scale Factor, example) use C#?
Take a look at this answer: how can I change the scale size of the FBX importer
Edit:
If you want to get the scale size for each resource then you have to identify it by its name or assetPath, maybe something like this (untested):
using UnityEngine;
using UnityEditor;
class MeshPostprocessor : AssetPostprocessor {
void OnPreprocessModel () {
if (assetPath.Contains("whateverString"))
myscale = (assetImporter as ModelImporter).globalScale;
}
}