Getting a sprites size in pixels

Orthograph cameras aren't too tricky as they don't scale with depth. You'll need to:

• Take the sprite bounds which are in world space (I was incorrect in my comment above)

• Use the camera orthographic size to get the size in screen space (-2 to 2 in x and y)

• Then scale that by the camera pixel width/height to get size in pixels

EDIT: I've added the 2nd approach that handles rotation.

Something like this:

         //get world space size (this version operates on the bounds of the object, so expands when rotating)
         //Vector3 world_size = GetComponent<SpriteRenderer>().bounds.size;
 
         //get world space size (this version handles rotating correctly)
         Vector2 sprite_size = GetComponent<SpriteRenderer>().sprite.rect.size;
         Vector2 local_sprite_size = sprite_size / GetComponent<SpriteRenderer>().sprite.pixelsPerUnit;
         Vector3 world_size = local_sprite_size;
         world_size.x *= transform.lossyScale.x;
         world_size.y *= transform.lossyScale.y;
 
         //convert to screen space size
         Vector3 screen_size = 0.5f * world_size / Camera.main.orthographicSize;
         screen_size.y *= Camera.main.aspect;
 
         //size in pixels
         Vector3 in_pixels = new Vector3(screen_size.x * Camera.main.pixelWidth, screen_size.y * Camera.main.pixelHeight, 0) * 0.5f;
 
         Debug.Log(string.Format("World size: {0}, Screen size: {1}, Pixel size: {2}",world_size,screen_size,in_pixels));
 

You can use the Sprite.Rect parameter, that stores height and width in pixel

I suggest something like:

var originalSpriteSize = image.sprite.rect;

     image.sprite = protoSprite.ToSprite();
     image.color = protoSprite.ToGameSpriteImage().Color;

     var newSpriteSize = image.sprite.rect;

     var scaleFactorWidth = originalSpriteSize.width / newSpriteSize.width;
     var scaleFactorHeight = originalSpriteSize.height / newSpriteSize.height;

     image.transform.localScale = new Vector3(scaleFactorWidth * image.transform.localScale.x, scaleFactorHeight * image.transform.localScale.y, image.transform.localScale.z);