try: page size 50.80cm x 28.57cm. It seems to work for me so far.
I created a full HD JPEG in Photoshop (1920x1080 pixels at 72dpi pixels per inch). Then created a new Impress presentation.
Right clicked the page and entered: slide > page setup.
At first I tried 16cm across by 9cm high and then doubled each in turn, until I reached largest cm size possible (300cm is largest so finally reached 256cm x 144cm) but when I inserted the full HD JPEG, the image was really small. so I selected original size and it was still really small. So I investigated the JPEG size with a right click and it was 50.80cm x 28.57cm) so:
Changed page size to 50.80cm x 28.57cm.
Then inserted picture. (have to right click > position & size and play with the settings a bit. ie: position x at 0cm and y at 0 cm. but then the image should fit perfectly).
I've not tested this as a presentation yet, but so far, this seems to be working.