gtk:cell-renderer-fixed-size (renderer)

renderer -- a gtk:cell-renderer object
width -- an integer with the width of the cell renderer, or -1
height -- an integer with the height of the cell renderer, or -1

The gtk:cell-renderer-fixed-size function returns width and height with the appropriate size of renderer. The (setf gtk:cell-renderer-fixed-size) function sets the renderer size to be explicit, independent of the properties set.