There are a few things that you can try to see if you can get the combo box to format how you want it. The first is to create the combo with gtk_combo_box_new_with_model(). That way you can setup your cell renderer and be able to set properties of how the combo box is going to be rendered. To help with the placement of the combo box widget you can use the widget expand and align functions. If you put the combo box in a box container then you have a few more things you can use for getting the alignment correct.