For a name like this, we can try a different approach, which is to pick characters that mean black and wood . With this approach, people in Japan cannot know how your name is pronounced, but they will know what it means instead. Usually, round hanko is used by individuals. Square hanko is used by organizations. There are exceptions.
Some artists have square hanko for their artworks.
If you would like your hanko to be quite large, we recommend rubber stamp. Yes, althouth this is not very common, there is a special kind of hanko, called Daihyosha-in . 18mm in diameter. If you are interested in this type of hanko, click here . I'm a Martial Artist Sensei.