ClarisImpact allows you to resize columns and rows by placing the cursor on a column divider in the first row of cells or a row divider in the first column of cells. When the cursor gets to this position it will change from a plus sign to an up-down arrow (for changing row height) or a left-right arrow (for changing column width) to indicate that you can now change the row or column size.
However, only the topmost portion of the first column divider and the leftmost portion of the first row divider make the cursor change from a plus sign into an up-down arrow or left-right arrow (See page 6-19 for a screen shot of the left-right arrow). The activation area is approximately 1/4" from the edge of the table frame.
The same is true for Timeline and Data Chart Models.
You can also click in a cell and choose Column Width... or Row Height... from the Table Menu to specify the row height or column width for that cell and its row or column. For example, to change the width of the first column, click in any cell in the first column and choose Column Width from the Table Menu.