...I know upsizing is not allowed...
Upsizing is not prohibited, it is not recommended. When you upsize an image in Photoshop, the resulting image usually becomes blurry or blocky, which is bad for printing. FAA tries to produce prints of a high quality and may regect an order if the downloaded image can not provide a good print of the ordered print size. FAA can not check quality of each uploaded image though, so promotion of your art, providing its printable quality and so on - it is mostly YOUR responsibility. If you can provide a printable quality of parts and the whole picture and hope to sell such panoramic pictures - you are on your own.