I'm new to Print on demans and I wanted to know if you have a way to resize the design in every size at once ?

When resizing poster image files to be uploaded for printshrimp orders, do we need to resize them to the Print Size in CM or Inches, or can we just upload a file that is the 'recommended pixel dimensions' size, and this will work fine? And also if we wanted, could we upload higher resolution images than the 'recommended pixel dimensions' size, and would this increase the quality of the poster, even if only by a small amount? :) I'm basing this information off of this page: https://www.printshrimp.com/blog/poster-aspect-ratios Thanks for taking the time to read :)