I've just re-tested, and you're right. This SVG won't display properly with chrome, firefox, inkscape or imagemagick
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg width="84000000000mm" height="84000000000mm"
viewBox="0 0 84000000000mm 84000000000mm"
version="1.1" id="svg5"
xmlns="http://www.w3.org/2000/svg"
xmlns:svg="http://www.w3.org/2000/svg">
<ellipse
style="fill:none;stroke:#000000;stroke-width:0.812697;stroke-opacity:1"
id="path111" cx="42000000000mm" cy="42000000000mm"
rx="42000000000mm" ry="42000000000mm" />
</svg>
However the browsers don't crash or run out of memory, so they've got that going for them. And if you embed the image in a web page with
<img src='84km.svg' width=1000 height=1000> chromium almost manages to display it.
SVG lets you specify sizes in a bunch of different units, but there's actually a fixed ratio between them - in SVG, 1mm means is 3.78px regardless of your display or printer's DPI.
So I didn't scale mm to SVG's arbitrary length units - but the standard did.