The Calibrator3D tool supports regular (same sized) squares in the calibration object as well as alternating dimensions. This is best shown by example:

Regular squares

Here, all squares in the calibration object are the same size, in this case, 30 by 30 mm. Therefore, Grid size odd and Grid size even are both 30, and Unit is "mm".

Alternating dimensions

In this case (example from Zhengyou Zhang at Microsoft), the black squares are 1/2 inch square, and the separating white space is 7/18 inch. The Grid size odd is then 0.5 (the black squares, grid number 1, 3 and 5), and Grid size even is 0.3888889 (the white dividers, grid number 2 and 4). Unit is "in".