"This school is big."
As said in other comments, 広い and 狭い are used when referring to the size of space while 大き and 小さい describe the size of objects. These are only easily confused because
A: We are used to describing anything large or small with "large" or "small" regardless of whether it's spaciousness or an objects size we're talking about.
B: The app doesn't do any explaining on certain things that would normally get an explanation in a formal school setting.
C: The app translates things into sentences like "the school is big" because that's what makes sense translated into English when literally the sentence translates into something like "The school is spacious" which would sound a bit awkward.
The point is, listen to comments that teach and pay attention to what's being taught in any lesson. If you're learning ひろい, then don't get confused because you aren't getting the option to use 大き.
Spacious might be a better way to think about in this context. As someone else pointed out on another question wide and narrow in regards to spaces tends to imply being large or small in one direction only (i.e. width). So big room makes more sense to me than wide room. I was taught the translation as both wide and spacious to help avoid confusion!