Monitors are getting bigger, phones are getting smaller... how does a designer decide what screen(s) to optimize for?
I never start by thinking of screen size. Usually my initial PS files are in excess of 4000px by 4000px. The aim is to ensure the content is right, relevant and actually needed.
After that process, when looking to get it into a workable form I let the (potential) audience dictate. If the majority are on mobile, then start there and scale up. On desktop, scale down.
Unfortunately, with all the focus on mobile the sad fact is 27in & upwards have simply been neglected. Not a big enough audience.