Keep sprites at fixed size

OK, that works pretty fine. However, I need to set "Pixels to Units" for every image manually? Is there some kind of method to automatically do it in script using formula? Because my application got 1200 images.

You can probably do it with an editor script. But considering the number of images, maybe you just want to scale the images at runtime. Assuming uniform scaling and a 'targetHeight' in world units, you can do:

 var bounds = GetComponent(SpriteRenderer).sprite.bounds;
 var factor = targetHeight / bounds.size.y;
 transform.localScale = Vector3(factor, factor, factor);
 

I don't know your game mechanic, but two other things to consider. If you can use a Quad for your image instead of a sprite, your problem goes away. And alternately (and not without some pitfalls), you would use a batch script in Photoshop to bring your images that need to be the same size to the same resolution.