ヒーロー画像ブロックの高さの「現在の値」の下限は20です。これよりも下の値にしたい場合はどうすればいいでしょうか？
You will find the minimum height of 20 is set in the concrete/blocks/hero_image/edit.php file on line 25
Thank you. I almost understood this solusion. If my understanding is correct，I have to make override folder, right?
An override folder in your application/blocks folder would be wise so that you do not risk the files being overwritten when you perform an upgrade of the core.
EDIT you will need to change the namespace in the controller file.