I don't know the 'right' way to do this but this seems to work.

After setting the count for rows and columns, right click on the shape and choose Group > Open Grid. This will open a window with the grids occupying a proportion of the space. You can lasso select the visible grid shapes and drag the group wider & taller to fill the visible size of the parent shape. Then close that window. You aren't constrained to even heights/widths and can resize rows/column shapes independently if you wish.

Note that I said 'visible grid shapes'. Before resizing the visible shapes, you can use Select All (Ctrl-A) and will see that the missing cells are still there, just invisible.

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Andrew Lockton
Chrysalis Design, Melbourne Australia