You can change the table sizes by accessing table properties; to do this open up the HTML editor in full screen mode, by clicking the "full screen" button.
In the main text area, select the table by clicking on it when the mouse cursor has changed to an arrowed cross. You can then resize the table by using the anchor points of the table.
Or once the table has been selected the table properties button is available on the HTML editor toolbar. From here you can change the width and height of each cell by entering a value on each of the Height and Width fields.