Actually the sentence "this office is big" is definite (i.e. the office is defined, by virtue of using "this") and hence in Arabic it would be: هذا المكتب كبير (al-maktab, the office). Thus, for the Arabic sentence above (where maktab is indefinite), the translation has to be "this is a big office"
Really????? Duolingo still insist on this Alif???
Sorry to tell you but no Arabic keyboard I've checked so far has this Alif by default - if I want to type it I have to use a third party help. Some iPhone or Mac users reported that they have it on their keyboard but for me on Windows and on Android never saw it.
This Alif is not used in the regular writing and typing and many people requested it to be removed. You would find it in Quran more but not in regular writing.
I guess the quickest solution I can offer you here is to copy it and paste it when needed!
This office is big: هذا المكتب كبير (háδá al-maktabu kabeer) [δ = TH in This].
When you say (This office) in English, you've actually "defined" the office by pointing to it with "this" - in other words, you do mean a specific office and not one of the offices. Thus, in Arabic, we have the combination هذا المكتب (this office).
However, if we say in English (this is an office), the Arabic translation would be هذا مكتب (and it is a full sentence). Notice how "AL" disappeared from المكتب became indefinite.
Hope it's clear.