Since you know your child best, pick an appropriate length based upon their ability in handwriting and their temperament. You may get the copywork done, but it is a waste if you loose the love of learning in process. However, on the other hand, you cannot let a lazy child rule the school.
You would do best to decide the level of content as well. You can use quotes above their understanding, perhaps that may even be preferred, but you must explain it in language your child can understand - so they know what the quote means. This is especially true of in quotes using an older fashion of language.
Introduction to Copywork - This is a link to a great article that gives you a great introduction
Site with many links on copywork
I am sure there are more, just