Add a button name Full Size on the pxUIGalleryModalDialog rule. For testing purpose, I modified this
via private edit. Configure the action set (Click >> Local Action) as per below screenshot. Here, I have
used the custom modal format name FullSizeCustom.
Identify the skin. For UI Gallery sample, pyUIGallerySkin is used. Refer any OOTB Modal dialog
component style(for example: fixed), saveAs this as FullSizeCustom.
Under the Window size, set the fixed size height and width as below:
Below screenshot shows the modal dialog is opened in full size.