You are over complicating

Set media size as you did.... 5 x 7
Now simplify:
Highlight select the thumb you want.
Now in Prints tab, click on the size print you want.
(You selected 5 x 7 paper)  So click 8 x 10.
Qimage will tell you how many sheets of 5 x 7 it needs to print an 8 x 10. (2x2)
Change size to 11 x 14, and now Qimage tells you that you will need 9 sheets of 5 x 7 paper to print 11 x 14 as a poster  (3x3)
After printing, you will lay them out and tape them together.
Fred