Post #351:  is:
bpy.ops.object.shade_smooth()
You need to copy that code and use it in Custom tab:
row = L.row(True); row.scale_x = 1.5; row.operator("wm.pme_user_command_exec", text, icon=icon, icon_value=icon_value).cmd = "bpy.ops.object.shade_smooth()"
Usescale_x value to change the size of the button.