Right-click the cell, click Cell Properties, Specify width/height in pixels
as necessary.
Note that if the content is too large to fit into it, like a whole strong of
letters or image that is wider than what the cell allows, the content will
force the cell to stretch horizontally
You can't. Table and cell dimensions are always a minimum. The cells
will always expand to accommodate the content placed in them.
If you haven't already done so, add widths to the cells. As long as the
content is able to wrap within that width, the cells will not grow
wider. Without widths, the cells will resize themselves roughly in
proportion to the content in each cell.
