Hi, I have a problem with the size of my files. When I create them I choose for exaple 20x20 cm and it seems like it will stay that way, but every time I check and open the file in PS it changes it's size by few mm and unfortunately it's bugging me a lot because my professors say it can't happen. What can I do? I feel like I tried everything and also I can't find reason for it to happen.
Work in entirely in pixels or use a resolution (ppi or ppcm) value that gives a whole pixel division for your image dimension. For example working at 118 ppcm instead of 300ppi gives exactly 2360 pixels for a 20cm wide document.
Working in a mix of cm, inches(ppi) and pixels is always going to lead to rounding errors. Working with cm , ppcm and pixels can avoid those.