It depends on both you and the artist tbh?
If you have a clear design idea in mind giving your artist as making details as you can to get the design properly translated on paper is important. If you only have a vague idea of something following a theme then you don't have to worry too much about giving lots of info. Then you need to find an artist that can work with what you have to offer. Some people prefer to work from short description with lots of artistic freedom, other prefer more in-depth descriptions with lots of reference pictures. Some people might get overwhelmed and stressed when getting too much info (especially if it's not in their main language) so if you got a detailed design idea in mind don't commission an artist that works best with artistic freedom and minimal descriptions.