You can change the size of the viewport of a textual region, or of all the textual region viewports in a class, with the change size action statement the same way you change a graphical region's size:  in X Y coordinates of its parent. (The Graphical Regions Changing the Viewport Size describes the change size statement.)

Note: Do not define or change a textual region's viewport size with the change size statement, unless you want it to occupy a given size relative to its parent.

It is important to note the visual effects produced by changing the size of a textual region's viewport. When you change the viewport size, the window size also changes (and vice versa), because text does not scale. Thus, a different number of lines and columns will be displayed in the changed textual region.

If you change the window or viewport size of the textual region's parent, the textual region is affected differently, depending on whether you defined the textual region by viewport size (size) or by window size. If you defined the textual region by the size specification, the textual region and its window will be changed accordingly. But if you defined the textual region by the window size specification, the appearance of the textual region will not change; it will still contain the same number of columns and lines.