32 cm wide by 25 cm tall.
Since each photo is square and has an area of 25 cm², that means that the length of each edge of the photos is 5 cm. So the length of a page (2 border gaps + 4 photos + 3 spacing gaps) is
2*3 + 4*5 + 3*2 = 32 cm
The height of a page (2 border gaps + 3 photos + 2 spacing gaps) is
2*3 + 3*5 + 2*2 = 25 cm
So the page size is 32 cm x 25 cm
