WPF Table Column Sizes

try TableCell.ColoumnSpan and TableCell.RowSpan

I do not think this is possible... the only hacky workaround is to use the BlockUIContainer and a real grid!

var fd = new FlowDocument();

Grid g = new Grid();

g.ColumnDefinitions.Add(new ColumnDefinition() { Width = GridLength.Auto });
g.ColumnDefinitions.Add(new ColumnDefinition() { Width = GridLength.Auto });

g.RowDefinitions.Add(new RowDefinition() { Height = GridLength.Auto });
g.RowDefinitions.Add(new RowDefinition() { Height = GridLength.Auto });

var t1 = new TextBlock() { Text = "ABC", Margin = new Thickness(5,3,5,3) };
t1.SetValue(Grid.ColumnProperty, 0);
t1.SetValue(Grid.RowProperty, 0);
g.Children.Add(t1);

var t2 = new TextBlock() { Text = "XYZ", Margin = new Thickness(5, 3, 5, 3) };
t2.SetValue(Grid.ColumnProperty, 1);
t2.SetValue(Grid.RowProperty, 0);
g.Children.Add(t2);

var t3 = new TextBlock() { Text = "123", Margin = new Thickness(5, 3, 5, 3) };
t3.SetValue(Grid.ColumnProperty, 0);
t3.SetValue(Grid.RowProperty, 1);
g.Children.Add(t3);

var t4 = new TextBlock() { Text = "789", Margin = new Thickness(5, 3, 5, 3) };
t4.SetValue(Grid.ColumnProperty, 1);
t4.SetValue(Grid.RowProperty, 1);
g.Children.Add(t4);

fd.Blocks.Add(new BlockUIContainer(g));