Mmm... often there is no difference. That room can be either suuri or iso (just as it can be either large or big).
Suuri is more often used when the large thing is abstract (suuri rakkaus (love); suuri päivä (an important day)). Iso is rather down to earth and concrete (iso koira (dog); iso mies (either a large man or a grown man or both)).
But apparently, iso is creeping into suuri's territory nowadays: https://www.kielikello.fi/-/suurella-sydamella-tai-isolla, and there were always regional differences as well.