Who understands the creation of a marketing kit?

Question

Marketing

I want to create a marketing kit for myself and in the lessons I read that the resolution with dimensions of 40cm × 20cm is 300 pixels (photoshop) under this quality condition, but it’s too big for me because the pictures do not fit the resolution. What would you advise what resolution to put on a brush with a size of 40x20cm. If you put 72 there will be no quality problems?