To do what you want to do, uncheck the thumbnail after adding one print to the Queue, then select the print in the preview, then click your various print sizes to see that single print change size.
John in Brisbane
John brings up a good way to approach it. Left Brain. Right Brain.
Qimage offers a priority to the left side when you select a thumb, and assumes you want action on it.
By selecting an image on the right side of the thumbs in the print screen, you are changing the priority to the the right side.
So John is saying, Deselect that left side selection, and switch to right side by selecting an image on the right side, in the queue.
... and yes, the Queue tab is showing you what Qimage is going to print when you click the print button. (I forgot that part in my earlier post)