To increase the height of several consecutive rows, try selecting the first row (clicking the row number), then hold <Shift> and select the last row. Then change the size of any selected row using the mouse.
To modify separate rows, select the first row, then hold <Ctrl> and select the other rows. Then change the size of any selected row.
To set the height according to contents, select the rows, then double click the bottom margin of a row.