Is Classical Chinese the shortest language?
it depends on the topic; style of writing, and translation. 

[for alphabetic language, one word might contain numerous characters, while for ideographic language, one word = one character]

when classical chinese is longer

"traduttore, traditore" (to translate is to betray) - 迻譯即背叛　⋯　2 vs 5 words

nautical terms, such as cutter, ketch, schooner; in which, one can grep its meaning and form an mental image directly.

in classical chinese, the word is longer

cutter - 雙頭帆小船　⋯　1 vs 5 words

ketch - 高前桅雙桅帆船　⋯　1 vs 7 words

schooner - 高後桅雙桅帆船　⋯　1 vs 7 words

or, buddhism terms, such as:

अनुत्तरा सम्यक् सम्बोधि (anuttarā samyak sambodhi) 
- 阿耨多羅三藐三菩提　⋯　3 vs 9 words

bhūtânityatā - 諸行無常　⋯　1 vs 4 words

bhūtânātmatā - 諸法無我　⋯　1 vs 4 words

śāntaṃnirvâṇam - 涅槃寂靜　⋯　1 vs 4 words

http://cbeta.kepan.org/sahant/7

http://cbeta.kepan.org/sahant/37

when classical chinese is shorter

well, chinese poem should be the shortest, amongst any languages.

taoism related text is, also. e.g. in 周易參同契:

金伐木榮．三五與一

cutting down by metal, the wood prosperous; three, five and one. (frankly, i don't think this book is "translatable"; i just give a word-to-word translation here, omitting background knowledges, assumptions; and implications)

https://ctext.org/library.pl?if=gb&amp;file=99940&amp;page=59

now, 8 classical chinese word vs 11 english words.


  when a lengthy document or a book is translated into different languages, will the classical Chinese be the shortest translation


imo, again, it depends on the topic; style of writing, and translation.

have fun :)