Width-In-Cells
This style implies that the value specified for the 
Size property, the one setting the Combo-Box control’s width, is expressed in CELLS. It is possible to get the same result writing: "
Size value CELLS".
 
Example - Define a combo-box in screen section with its width in cells
screen section. ...   03 screen-1-co-1 Combo-Box      line 19.0      column 23.0      size 12.0       lines 11.0      id 13      width-in-cells      3-d      drop-down      .  |