How to set the height of the button of a combo box ?

[水 丘岑 <sqc_zz hotmail com>]

When I write as follow:
GtkWidget *list_combo=gtk_combo_new();
gtk_widget_set_usize(list_combo, 146, 34);
There's no changes.

And with these:
gtk_widget_set_usize(GTK_COMBO(list_combo)->entry, 146, 34);
I only can change the height of the entry.

with these:
gtk_widget_set_usize(GTK_COMBO(list_combo)->button, 146, 34);
The only change occured on the button's width. And how can I change the 
height of the button on the rigth?

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