I wrote "are those big?" And Duolingo days that the correct answers is "are they big?". Why my sentence is not correct? I thought that "Jsou" cloud mean either "they" or "those"
You are right that jsou TO can mean both "they are" and "those are." But here -- without the TO -- jsou just means "they are."
So in this case, is "they" a bunch of neuter nouns?
No, neuter would be "Jsou veliká?" in standard Czech. Here it is feminine or inanimate masculine.
Would someone please explain what is the difference between "veliké" and "velicí"?
Veliké is feminine, velicí masculine. Velicí muži, veliké ženy, veliká děvčata.
That makes sense, thank you for the explanation!
Is velké an alternative form of veliké? If yes, which one is more common?
Yes, it is an alternative. Velký(á,é) is more common. http://syd.korpus.cz/aTzxo1wb.syn
I still don't quite understand when i should use veliká or velká
They are equivalent and both belong to feminine singular or neuter plural.