The first opening refers to the original worksheet.
The second opening is divided into two pieces at a time.
The fourth opening is divided into two times and changed into four times.
etc
Generally, the first opening is 787* 1092mm, some are larger and some are smaller, so the same opening is 16 but the size is different.
So 16 is twice as much as 32, and 32 is half as much as 16.
The first opening size of international standard is about 860mm *1120mm = 0.96m2. The first opening size of China standard is about 740mm *1040mm = 0.77m2. The above figures may not be particularly accurate, but they are basically the same.