Choosing the right size for a portable projector screen depends on several factors.
First, the throw distance of the projector, which is the distance between the projector and the screen. The throw distance will determine the minimum and maximum screen size for the projector.
Second, the aspect ratio of the projector, which is the ratio of the width to the height of the projected image. It is important to choose a screen with the same aspect ratio as the projector to avoid distortion of the image. The viewing distance is also a crucial factor to consider, as it will affect the perceived size of the image.
Finally, the intended use of the screen, whether for home theater or presentations, should also be considered.