Override wpclsQuerySettingsPageSize (or some such verbage, I don't have docs at home). I don't know if this API made it into the on-line docs, but it is definitely public (see \TOOLKT20\C\OS2H\WPOBJECT.H). There you are given the opportunity to adjust the size up to accomodate your dialog, if necessary.
The PAGEINFO structure is used by the Settings page code to delay the loading of the dialog required for a page until the user turns to it for the first time. It has the resource ID, module handle (which is incorrectly named in the structure, 'resid' or some nonsense, grr-r), dialog procedure, and pointer of your choice to receive on WM_INITDLG (most choose the object pointer, somSelf). When the user selects the notebook tab, WPS calls WinLoadDlg based on the parameters in PAGEINFO.