You can only change row heights and column widths. Not individual cells.
However, you can merge cells to make them appear the way you want. Try
highlighting a couple of cells that you want to merge, and then right click
them. Select Format Cells. On the Alignment tab, check the box for Merge
cells.
I'm pretty sure that is not possible.
What do you intend to do with the "shortened" cells. is it for printing?
formatting?
an alternative would be to Merge two adjacent cells to form the "longer"
cells and leave the other cells as single cells. You could then change the
widths as needed.