You can stretch or shrink cell borders by moving your mouse over the relevant border. When the pointer changes form depress the left mouse button and drag the border to a new position before releasing. When you adjust cell width with the mouse, the adjustment is made for the entire column by default. If you want uneven cell widths, use the Ctrl key as you move the cell border (or set the width using the Cell Height and Width dialog available from the Table menu).
By default, the cell height in a table is sized automatically to fit the amount of text (or graphics) you have in the cell. If you want fixed height table rows or want to set the row height exactly, select Cell Height and Width from the Table menu, clear the Auto Height checkbox.