The compile error you get is because you try to pass a QSizePolicy::Policy to setSizePolicy(QSizePolicy), but there’s no implicit conversion from QSizePolicy::Policy (which is the policy for one dimension) to QSizePolicy (which is a class containing, among other things, one Policy per dimension (height, width)). QSizePolicy doesn’t work on top-level widgets (windows) anyway, though.
setFixedSize() only works if you know the size of the dialog in advance (and usually you don’t, what with changing font sizes and languages). You can do
window()->setFixedSize( window()->sizeHint() );
but it’s much better to use
window->layout()->setSizeConstraint( QLayout::SetFixedSize );
That lets the layout determine the size of the dialog, but doesn’t allow resizing, which I assume is what you were asking for.