Actually it was SETpredefinedFrame.
And also the effect sometimes took effect sometimes don't.
I have trial and error around and by doing
m_DockManager.DocumentStyle.DocumentViewStyle = DocumentViewStyle.MdiStandard
change away from MdiStandard and then change it back again - will update the document frame.
However, doing so I lost position/size of mdistandard document windows.
So I am going to open different question.
I thought i have solved it but if i turn on skin e.g. macskin...
When change to another skin the mdistandard doc will still be in Macskin frame