To add to Lamprey. In photoshop, you don't need to work in pixels, either. You can do everything in MM or inches. Say you are making a rectangle shape and it asks you for the dimensions, it will probably default to 100 px by 100 px. In the type box if you want 15 inches, you just type 15in and it will convert. Same goes for typing in mm.
You also have rulers at your disposal. You can go View > Rulers, and a guide will show up. around your image showing you the size. Then you have the ruler tool. By default, this should be one of the tools that shows up when you click and hold on the Eyedrop tool. It will show you the size of any shape you make.
Did you follow plans to make your cab? I don't know your level of photoshop, but here a basic, practical method I would go with if you have the measurement image.
1. Make a new image that is a bit bigger than your bartop dimensions. So if your bartop has a side dimension of 15 inches x 18 inches, I would probably go with a new 20 in x 20 in image. (you can crop later). Set your resolution to 300 dpi to 600 dpi max. The bigger the dpi, the bigger the image.
2. Grab the measurements image of your bartop. If you have an image like this with the actual measurements on it, that would be ideal.

3. Make a rectangle shape that is the total dimensions of your side art. In the example above, you would make the rectangle. 570mm wide and 950mm tall.
4. Now paste in your measurement image. (Hope that it is "to scale").Transform the size of the measurement image to meet the rectangle size you made. You can turn down the opacity of the layer if you cant see behind the image. Now is time to find out if your measurement image is to scale. Pull out your ruler tool and start each dimension.
5. If your image is to scale, meaning all the dimensions line up with their measurements, then grab the pen tool and trace the outline of your cabinet. If it is not to scale, you will need to tweak your pen tool lines. Once your cab is outlined with the pen tool, you can delete the measurement image and the rectangle you made earlier.
Now when you photoshop your side art, make sure you leave plenty of bleed, or extra art that spills past the outline you made.
You can also make an outline from scratch in photoshop if you don't have a measurement image. You will just need to make basic shapes using the shape tools and the pen tool and put it together piece by piece.