ThumbViewer.AutoSize
From Jeremie Leroy - XOJO Controls Wiki
Property (As Boolean)
If the ThumbViewer is a HorizontalViewer and ThumbHeight + 2*ThumbSpacing is greater than the Height of the ThumbViewer, the ThumbNails will be resized to correctly fit. For Vertical ThumbViewer if the width is smaller than ThumbWidth + 2*ThumbSpacing, the Width of the Thumbnails will be resized.