I just tried "široký strom" as the translation of "large tree" and reported it since it was not accepted. There is a pragmatical difference between "big" and "large" in English, which is somewhat mirrored in Czech as the difference between "velký" and "široký". Of course, meanings overlap pragmatically, and thus "široký" should be accepted.
Point taken dude :) But Czech overlaps meanings of "wide", "large" and "great" with the various forms of velký, veliký and široký depending on the actual point of discussion. Exempli gratia, "To je fakt velký strom!" could be translated into English as, "That's a really big tree!" as well as "That's a really large tree!", depending on the pragmatical meaning of the utterer... Simply... yes, you're right... there are more possibilities to any utterance than the word-for-word translation, and these can lead to confusion until the learner does learn the subtle differences between meanings. Therefore, more translation possibilities could naturally be considered by Duolingo Contributors. I may remain to be corrected, however... :)