This option is analogous to HEIGHT-IN-CELLS, except that it affects the width of a control. This style is particularly useful when you want to center a label over a region of the screen. By specifying the label's width in cells, and using the CENTER label style, you can easily do the centering without knowing anything about the label's font. For example, if you have a window that is 40 columns wide, and you want to center a title in it, you could use:
DISPLAY LABEL MY-TITLE, COLUMN 10, SIZE 20 CELLS, CENTER
Assuming that the title fits in 20 cells, this works for any title in any font.