For each OGF file, there is an image at full size. Each other mipmap in the OGF file is half the size of the previous one. This means the second image would have half the width and height of the first, and the third would have half the width and height of the second.
For example, the largest image in a particular OGF file is 100 x 100. The second image in this OGF file therefore must be 50 x 50. The third must be 25 x 25. There cannot be a fourth mipmap since 25 is not divisible by 2!
For more information, see the OGF Editor manual.