To quickly change a row or column dimension, drag the right edge of the column header or the bottom edge of the row header to a new location.
Change All Rows /Columns
To change all rows or columns at once, click on the unmarked corner header at the upper left (top of row 1 - left of column A), which will highlight all the cells. Then select Row/Height or Column/Width from the Format menu.
Change Dimensions by Number
To change the row and column dimensions by using a precise number, right click the row or column header and select Row Height or Column Width.