The longer your sentence, the more control you need over punctuation, linking words, and use of pronouns. If you use any of these badly, then your reader will become confused. The last thing you want is for your tutor, getting through a pile of 50 essays at midnight, to have to start reading a sentence three or four times to try and make sense of it. It is therefore usually recommended that you write in fairly short sentences, maybe a maximum of around 20-25 words or a couple of lines. As you get more practice and your writing improves, you can start to make your sentences longer.