What you really need to match is the angular resolution in microradians from your eye. You can make any screen smaller by sitting farther back. That said, I do wish my TV was only 42". I guess if you really want the ppi to be exactly the same as a 27" 5K screen, then 27 * 7680 / 5120 = 40.5".
This is exactly the reason I intend to stick with 4k for now: I don't want a display that large. I currently have a 48" 4k display, and I'd prefer to have a 42" or 36" one. (Good choices are hard to find, though, particularly if you actually want 4k rather than ultrawide, want OLED, and don't want to just use a TV.)
I bought the Philips Evnia which fits perfectly into that category at 42". Despite being a gaming monitor it's not garish and I've grown to love the ambilight.
