"I see a big school."
Translation:Én nagy iskolát látok.
Good question. The difference I see between the two sentences is that "(egy) nagy iskolát" is emphasized in "Én (egy) nagy iskolát látok", whereas in "Látok egy nagy iskolát" it isn't. So omitting "egy" is possible in an emphasized phrase, but unnatural in a not emphasized one. But this is only a guess, so if anybody who knows better is reading this, please confirm or correct me.
Anyway, it might be better to stick to "egy", as most of the time we don't drop it, even if it's possible to do so. This sentence sounds a lot better to me as "Én egy nagy iskolát látok" than the preferred translation.