Setting relative size of a widget is based on the widget's parent container, the cell, not the form. To set the size based on the form, you need to set the appearance of the cell first and then the widget. Some widgets are not supported: see Supported XUI elements for relative size. Here takes setting the width of TextArea as 25% of the form for example.
1 Select the widget and turn to Cell Appearance tab.
2 Check relative and set the percent value as 50.
There are two cells on horizontal direction. To set the width of TextArea as 25% of the form, it is better to set the cell as 50% of the form. And TextArea would take 50% of the cell (25% / 50%).
3 For widgets inside container, you need to set container's relative size first. And for content panel of TabbedPane, there is one more step.
▪ To set relative width, set FillHorizontal to true.
▪ To set relative height, set FillVertical to true.
4 Then turn to Appearance tab and set relative percent to 50%.