vmin
One percent of the viewport's smaller side — the square that always fits.
One vmin is 1% of the viewport's smaller dimension: the height on a landscape screen, the width on a portrait one. It is the unit that does not need to know the device's orientation to promise it fits.
It is the right choice for what must stay whole under any rotation: a square badge of 20vmin, a circular dial, a cover title that never overflows — the element hangs from the tighter side and leaves the other alone.
.selo {
width: 20vmin;
height: 20vmin;
}