The LaTeX source may be easily edited to impart any size/proportion desired --- not quite as straight-forward as re-sizing a browser window, but the same ultimate effect.
Not really. It's fixed at "compile time". Eg "A4" or "Letter". I guess it "changes" according to zoom level, but that's just enlarging or shrinking - no reflow required.
How long does a fast latex renderer take on a large document? It's been a few years since I worked with latex, but it's on the order of multiple seconds, right?
https://courses.umass.edu/klement/imp/
which is available in a number of sizes _and_ has the LaTeX source available showing how the various sizes are arrived at.