Hi, I'm Shamim Hossain
Answer some questions and I will help you to figure out the right CSS unit for your project.
Assuming you are only using these units for web development, not for print media.
max-width + px rem or %
Flex items, by default, will not grow, so normally you only need this if you have declared a flex-grow other than O on your flex items. In that scenario, if they are allowed to grow, but if you also declare a max-width, they will not be able to grow bigger than that size.