Select the rows you want to resize. You can click on the numbered heading to the left of the first row you want to adjust and "Shift-Click" on the heading for the last row in the series. Alternatively, click and drag through the range of headings to make a multirow selection.
Click on the "Format" item in the Home tab's Cells group. In the Cell Size section of the drop-down menu that opens, click on "Row Height."
Type the desired size into the "Row Height" field of the Row Height dialog box. Click the "OK" button to apply your changes.