When a bitmap is used on a push button, it does not care for the cells. The size of the PB seems to be fixed and cannot be changed via Acubench. It is not possible to get the "lines 20 CELLS" in the source.
ACUCOBOL-GT User Interface Programming Version 7.2
Chapter 5: Control Types Reference > 5.15 Push Button >
5.15.1 Push Button: Common Properties
When the BITMAP style is used, the LINES and SIZE values have a different meaning. The values are the number of pixels in the height and width of the bitmap image (see section 3.7, "Bitmap Buttons," for details). If omitted, the default values depend on the host system. Under Windows, the default LINES value is "15" and the default SIZE value is "16". These correspond to the size of buttons typically found on a toolbar.