The sizing method the tree view column uses to determine its width. Please note that the value :autosize is inefficient for large tree views, and can make tree view columns appear choppy.

(define-g-enum "GtkTreeViewColumnSizing" gtk-tree-view-column-sizing
  (:export t
   :type-initializer "gtk_tree_view_column_sizing_get_type")
  (:grow-only 0)
  (:autosize 1)
  (:fixed 2))  

:grow-only

Columns only get bigger in reaction to changes in the model.

:autosize

Columns resize to be the optimal size everytime the model changes.

:fixed

Columns are a fixed numbers of pixels wide.