I'm not sure that what you are asking is the best way to do what you want. I'd think if you don't actually want to show the CDialog, then you'd create the CSizingControlBar in some other parent window and use the handler there. In any case, to answer your question:
If you don't want to call DoModal() on your dialog, you'll need to create the window using Create():
http://msdn.microsoft.com/en-us/library/yhth57kd(v=vs.90)[
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Typically for a non-modal dialog, you'd call Create() and then ShowWindow(SW_SHOW). See the example text in the link above.
However, you could call ShowWindow(SW_HIDE) if you want the dialog to be hidden. (You'd have to call ShowWindow on the child control I think.)