By design the  MinHeight property is not respected in ColumnGroupsViewDefinition.

The right way to apply MinHeight in the ColumnGroupsViewDefinition is setting the GridViewColumnGroup's RowSpanProperty and GridViewColumnGroupRow's MinHeight property. For example:

            this.columnGroupsView = new ColumnGroupsViewDefinition();
            this.columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup("General") { RowSpan = 40 });
            this.columnGroupsView.ColumnGroups.Add(new GridViewColumnGroup("Details") { RowSpan = 40 });
            this.columnGroupsView.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Address") { RowSpan = 40 });
            this.columnGroupsView.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Ime Tam") { RowSpan = 40 });
            this.columnGroupsView.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow() { MinHeight = 40 });             
            this.columnGroupsView.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow() { MinHeight = 40 });
            this.radGridView1.Columns["ContactName"].RowSpan = 40;
            this.columnGroupsView.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["CustomerID"]);
            this.columnGroupsView.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["ContactName"]);         
            this.columnGroupsView.ColumnGroups[0].Rows[1].Columns.Add(this.radGridView1.Columns["CompanyName"]);
            this.columnGroupsView.ColumnGroups[1].Groups[0].Rows.Add(new GridViewColumnGroupRow() { MinHeight = 40 });             
            this.columnGroupsView.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["City"]);             
            this.columnGroupsView.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["Country"]);
            this.columnGroupsView.ColumnGroups[1].Groups[1].Rows.Add(new GridViewColumnGroupRow() { MinHeight = 40 });          
            this.columnGroupsView.ColumnGroups[1].Groups[1].Rows[0].Columns.Add(this.radGridView1.Columns["Phone"]);