Long or short?
Long words or short words
The most important aspect to consider when you're writing is not whether you're using long words or short words, but whether your writing is clear. In academic writing, you will naturally find that you're using technical terms, and these will often be long words. Take every opportunity to learn these technical terms - read widely to see how they're used in context.
But don't write long words to try to sound impressive, especially if you're not sure of their meaning. You'll just end up confusing your reader, and probably yourself too.
Long sentences or short sentences
What we said about long and short words applies to sentences too - it's more important that the sentence is clear and understandable than how long it is. Academic writing tends to use long sentences, but it's also a good idea to vary long sentences with short ones. A short sentence coming after a few long ones has a lot of impact.
Sometimes you can use a short sentence to give an illustration of what you've just said in a long one:
"In many cases, however, despite the existence of general conventions
it remains a writer's personal decision whether to use a comma or not.
Some people like using commas and others prefer not to." (Fabb and Durant, 2005, p.147)
Sometimes you can use a short sentence to contrast with what you've said in a long one:
you need to use someone else’s ideas you normally paraphrase rather
than quote, because this is a way of rewriting the relevant material
in a way that assimilates it into your essay and your own voice. In
contrast, a quotation leaves the incorporated material sticking out
because it is in a different style." (Fabb and Durant, 2005, p.129)
So, as both long words and short words have their uses in academic writing, so do long and short sentences. And mixing them together makes for lively writing.
Fabb, N. and Durant, A. (2005) How to write essays and dissertations: a guide for English literature students. 2nd edn. Harlow: Longman.