One may cut a brownie that is larger than the minimum dimension as long as the overall dimensions are multiples of the minimum dimension. At no time may the main brownie mass be cut to a size that doesn't confirm to the grid. This can lead to confusion for future brownie cutters and often results in a chain of violations.
The grid size is determined by the baker of the brownies. If the baker fails to establish a proper grid size verbally or by failing to make the first cut, he/she forfeits the the grid size decision to the first person to make the cut.
Brownies may be eaten that are smaller than the minimum dimension as long as a piece meeting the minimum dimension is severed from the main brownie mass first. Once cut to the minimum dimension, the piece may be cut into smaller sections and distributed as you see fit.
See Figure A for examples of valid and invalid brownie cuts.
Note that the brownie rule may also be applies to cakes, pies and any such treat that may be cut to even sizes. As a courtesy, you should always make you guest know when the brownie rule is in effect.