Start with Sketch ONLY and getting your idea or brief right by revision and feedback, for you to approve, after that you will need to order to have the sketch finalized into final illustration.
The standard Illustration specification is: A4 print size and resolution (2480x3508). Reasonable amount of details and number of characters in one piece.(More characters = less detailed, and vice versa). Intricate and bigger illustration cost more gigs/extra.
Concept sketch process is required, we can't guarantee the best result without it. Final revisions are for free if I misinterpret your brief or made mistake on the illustration.