The most basic form of aspect ratio is a perfect square. The aspect ratio of a square would be expressed as (1:1), meaning that the height and width are exactly the same as each other. The ratio will remain fixed, regardless of the size of the square. It makes no difference the square is 100mm x 100mm, 500mm x 500mm or 20 x 20 metres.
The relationship between the height and width always retains the same proportion. Therefore, the figure of (1:1) will always remain the same so long as it is still a square. Whilst the height and width remain equal, with the same unit of measurement on each side, your large square photo prints will always be (1:1).