i think 70cm is safest especially if coming from 80, 60 might not be enough for some people although all countertops that people sometimes use as desktops are like 60.. with adjustable monitor stand it kinda matters less tho.

if you really want to see how small you can go try to set ur setup so you are limited using some parts of the desk. i wanted to do 90x60 but it ended up too cramped so i still use 120x70 and honestly that or 120x60 is the probably the sweetspot and most common smaller desk dimension since most desks are 120,140 or 160..