Here's what you have to do:
Groomer: So what kind of cut are we doing today?
You: Well, I'd really like ______(insert style of cut here, be it a schnauzer cut, same length all over, etc.), but I think I'd like to grow out the beard, so don't take too much, if any, length off there.
Groomer: Okay.

We're here to provide a service to you! Just tell us what you want and we'll do our best to accommodate.