Question : Is there a preferred size to illustrate children’s books?
Answer : I’ve found is that size varies from publisher to publisher. Happily, if your manuscript is picked up by a publisher, they will let you know what sizes are available to you. This goes the same for self-publishers, who are able to give a good rate to authors for printing books because they have a standard cookie-cutter size they work with.
A very helpful editor over at Omnibus Books gave the advice at a children’s book writing seminar that it is very unusual to receive finished illustrations with a manuscript. She went on to say that it is even discouraged because of the uncomfortable situation that can arise when the story is literary genious and the illustrations are scribble, or the illustrations are masterpieces and the story is dribble!
With that advice in mind, if you wish to submit a manuscript with illustrations, I would lean towards only sending in sketches and perhaps one finished illustration (as a sample) with your manuscript to a publisher.
That being said, there are many talented aspiring writer/illustrators out there… and to you I would give the advice to just illustrate larger than you hope the book will be, and paint extra bleed (trim) around the illustration. That way if your story is picked up by a pubisher, there is flexibility to scale down and trim the illustrations as needed.
You can find more tips on how to illustrate a children’s book here. And if you have any questions about children’s book illustration or publishing, feel free to post your questions in the comment section.