The recommended crochet hook size for DK yarn is generally 3.5-4.5mm. However, its important to note that these are just recommendations and not set in stone. Using a larger hook size than suggested will give you looser, more open stitches, as well as making your finished crochet piece larger. Using a smaller hook size will give you tighter, denser stitches, and your finished piece will end up being smaller. Ultimately, the hook size you choose will depend on the effect you want to achieve with your project.