Hi Hamid,
You can set the Group's IsCollapsible property to false to prevent it from collapsing at all.
The two buttons after the separator would need to be in a ribbon:StackPanel. That should keep them large and side-by-side by default but then move them to being on top of each other when space is needed. The StackPanel.ItemVariantBehavior property governs which variants are applied within the stack panel and when.