Here’s how to turn one label that spans several columns into a set of short ones, “stacked” in one column, but spanning two or more columns:
Select the cell containing the long label and one or more cells to the right of it (make sure the cells in the rows just below are empty).
Select Fill from the Edit menu, then Justify from the submenu.
Excel breaks the long label (by removing spaces between words) into short ones, and enters them in the first column, starting with the cell that contained the long label. The long label is broken into pieces long enough to fit into the number of columns you selected in Step 1 above.
If you change the width of the columns after you stack the labels, you may want to refit the labels. To do this, first select all the cells that contain text as well as the cells in the adjacent columns. Select Fill from the Edit menu, then Justify from the submenu.