Making a 50cm*80cm poster with Photoshop depends on whether your poster contains pictures or not. If no image is included, a new image of 5cm*8cm is suggested, and then it is given to the printing unit for scaling up. Enlarging only the text does not reduce the resolution.

If it contains images, it will be troublesome. A 50cm*80cm image has nearly 60 million pixels (300 pixels) and is in four colors (CMYK). If you make it layer by layer, it is estimated that the average computer will be dragged to death. If your computer has enough memory, help yourself. If not, find a printing unit to do it for you, which is usually free.

If you want to do it yourself, look at Responder's answer: ccmlove. These are several key issues.

Nothing else.