The suggested English translation, 'There is a big square', is weird. Better would be:
1. A big square is there.
2. There is a big square there.
I think #2 is the best English translation, using one 'there' for location (ott) and the other for the idiomatic English filler word.
And note that in #2 it is definitely the second 'there' that translates the 'ott', not the first. That's part of the reason that the DL suggested translation sounds odd.
One time I wrote large, and it corrected it as big, then the very next sentence used 'nagy' again so I wrote big, and it corrected it as large. Very frustrating.
I think it is just random depending on how they wrote the english translation, so we should just keep reporting them until they all accept both options.
No reason. I suppose there may be some very isolated cases where one sounds much better than the other, but in general, both "big" and "large" should be accepted. In every case I see "nagy" in a sentence, I try both "big" and "large", and report it when either is not accepted. I recommend that everyone does the same.
Remember that this is still an early Beta release of the course, only out a couple of weeks ... there are going to be many mistakes like this, and the developers want our feedback.
Some nouns will have their (last) vowel shortened in plural: víz-vizek (water/s), tér-terek (square/s), szél-szelek (wind/s), tehén-tehenek (cow/s), etc. Most retain it unchanged: só-sók (salt/s), ház-házak (house/s), szél-szélek (edge/s), méh-méhek (bee/s), etc. Some will lenghten their last stem vowel: kutya-kutyák (dog/s), macska-macskák (cat/s), bika-bikák (bull/s)... And, finally, some may even change their last stem vowel: borjú-borjak/borjúk (calf/calves) [the first alternative is the traditional, the second is a newer form], hó-havak (snow/s), tó-tavak (lake/s),... I think, the best approach is to learn them all by heart.