Using Tab control to change between dialogs

I've created a tab control wich I want to use to change between diferent dialogs. When I select a tab I will just show it's corresponding dialog in the tab control's client area.
But what I can't work out is how to set the correct dialog size:
I know the size of the tab control so the logical thing to do would be to put that size into the resource script for the dialog. But I don't think the numbers in a resource script correspond to width/height in pixels they all represent dialog units. Dose anyone know what the best thing to do for this creating a gui like this?

Use GetClientRect() on the TAB ctrl. GetWindowRect() on the dialog.

SetWindowPos() to change one of the sizes.

Better to make one dynamicly (on the fly) rather than use a resource script (if the dialogs are not to complex).

Thanx for the help novacain