I tried to use a 30" 4K screen at nominal resolution, but the type is too small for me at normal distances. I find that the sweet spot is a 40" 4K TV. If it's curved, it's slightly better.
In theory, you should pick a 5K display, but there only a bunch of models to pick from and they cost a lot (some are not even manufactured anymore)
You should always assume that you'll be scaling your image at 2x when using high ppi displays. Take the iMac 5K for example - it offers the same screen real estate as a 2560x1440 display, just twice as sharp at 220 PPI.