"We have a large basement."
Translation:Meillä on suuri kellari.
OK, I researched the subject. "Suuri" is used when talked about something great, like abstract ideas, emotions, something grand, magnificent, (like "this is a great day", "he is a great man", "he has a big heart") "iso" is used when talked about something big of size, something concrete, something that is large (like "it was a big bear", "he's a big man") So - iso would be more suitable here.
It seems that somebody just really likes the word "suuri", it's all over the Finnish version, from "suuri maa" to "suuri sieni". It's shruggable when both "suuri" and "iso" are accepted and the notes just point you to "suuri" being the "righter" one, so to speak; but when "iso" is rejected, it stops being funny. And it is still rejected, 2 months after the first comment here.