public class MyLayoutConnector extends GmfDiagramLayoutConnector {
    ...
    protected KNode createNode(LayoutMapping mapping, ShapeNodeEditPart nodeEditPart,
            IGraphicalEditPart parentEditPart, KNode parentKNode, Maybe<KInsets> kinsets) {
        KNode node = super.createNode(mapping, nodeEditPart, parentEditPart, parentKNode, kinsets);
        if (nodeEditPart instanceof MySpecialEditPart) {
            KShapeLayout shapeLayout = node.getData(KShapeLayout.class);
            shapeLayout.setProperty(CoreOptions.NODE_SIZE_CONSTRAINTS, SizeConstraint.fixed());
        }
        return node;
    }
}