Or is a 4k display good enough? I'm not sure I have the tools to work this out mathematically but we can start with some basics. I'm also not sure about the effects of distance vs resolution.
Tree leaves in a canopy are going to be our example.
An HD display is 1920x1080. Displaying a 15ft wide canopy on an HD display results in 94mil/pixel (15*12*1000/1920) across the width.