"We will have a big appetite."
Translation:Ni havos grandan apetiton.
Certainly a case could be made for "grandajn apetitojn" if we think that each person has his or her own appetite - rather than one collective one. My suggestion is that if you encounter a similar sentence in the course, that you express your translation as plural if it's plural in the original and singular if it's singular in the original.
Personally, I would think it weird for someone to speak about my appetite on my behalf like that.