I let wxSmith generate the SplitVertically function with 2 panels as parameters, then edited my *Main.cpp file to add the third parameter to an acceptable value, saved, compiled, and ran, and saw no change in the sash position. I have many components populating the left panel, and only a TextCtrl in the right panel. I suppose, since it worked for you, that there may be some other wxSmith property that I am not setting correctly, but I like the SetSashPosition function since I know it works regardless of other settings.