It depends on numerous factors. For starters, it depends on the resolution of your image file. We automatically check this after your upload, so that you are only shown formats that will work with your image file. Additionally, the viewing distance to the picture on the wall is important. Our general rule of thumb: the viewing distance = twice the diagonal measurement. The further viewers will generally stand from the picture, the larger it should be. Conversely, if the picture is going hang in a narrow hallway, you should not make it too large, since the viewer will not be able to step back and take it all in. In a small hallway, it would make more sense to have multiple smaller pictures. For pictures above the sofa, however, you should not pick too small a size, since the viewer will always be at a distance. For this a picture can easily be 100 cm or more.
Here are a couple of examples to get you started:
With our RoomView feature, you can virtually hang your photo on your wall in various sizes for testing via our website.