Here's how I've been doing it:
1) Draw everything to scale in QCad
2) Take a screenshot of it
3) Crop the image to the precise borders
4) Stretch the image to the full size I need (in inches) in a paint program
5) Save it as a bitmap
6) Use the layout as a guide for where I draw my graphics

Print it out in Photoshop (with exact dimensions)
Qcad is an open source 2D cad program. I've never used cad at all before making a control panel, but I thought I'd try it. It's awesome! Probably QCad isn't the best cad program out there, but it does what I need it to do. It's a lot like Corel Draw, only built for the purpose of using exact dimensions.
I suppose that if you know what you're doing with Corel or Illustrator or Photoshop, you can have the same effect, but I'm finding this to be really simple.