Page 88 of the reference given above by @T.E.D. gives:
The author further observes, on the following page, that:
The most suitable brick shape that can improve the way of joining the structure to make it stronger, as well as the ease of transport from the production site to the construction site, ... were important elements that played a main role in determining the dimensions, size, and shape of the brick.
While some patterns gradually appear in the aspect ratios, as noted on page 91 of the reference, full standardization is not seen until the Neo Babylon period some centuries following collapse of the Indus Valley Civilization about 1300 B.C.
Thus the wide variance (across both time and space) in size and shape, in width from 12 to 23 cm and in width:thickness ratio from 12:6 to 23:6 for example, belies any concept of there being a "standard" brick dimension. With the failure of such existing in Ancient Mesopotamia, the suggestion that such a standard might have been transferred to the Indus Valley by trade becomes unfortunately meaningless.