Regulations of Aircraft Registration Article 25
The height of each letter of markings on free balloons and airship shall not be smaller than 50 centimeters.
中文原文 航空器登記規則第25條
自由氣球及飛艇之標誌，其每字之高度，不得小於五○公分。
English translation for reference only; the Chinese text prevails. Not legal advice.