How To Do Posters
Photoshop and other photo editing software products use crude, dated methods to perform this “interpolation” that was being used in software decades ago. These interpolations methods are known as the “nearest neighbor,” “bilinear,” and the “bicubic” interpolation methods.
Read more about these three interpolation methods here.
To make posters that look professional you should use software specifically designed for enlargement or you can get “blocky” or “jagged edge” parts of your image, especially where edges and colors change abruptly. There are several photo enlargement products available - search for “photo enlargement software” in any search engine. Be aware that you should not use photo editing software like Photoshop for photo enlargement. Photo editing software was not designed for enlarging photos.
Before using photo enlargement software (or Photoshop if you do decide to go that direction) you need to measure the space where you will be displaying your poster. Simply measure the height and width of the space you want to cover and match those dimensions in the software.