テクスチャのプロパティー名から画像の大きさを取得します
Common texture names used by Unity's builtin shaders:
"_MainTex" is the main diffuse texture. This can also be accessed via mainTextureScale property.
"_BumpMap" is the normal map.
"_Cube" リフレクションキューブマップ
See Also: mainTextureScale property, SetTextureScale.