How do I change from mm to inches in Page Setup for printing specal sizes?

Had the same issue in PRINT JOB ... the sizes were in inches and I needed them in cm.
The solution is easy ... scroll up in Lightroom, to the LAYOUT tab, and make the change there. 

Progress is being made. Now the question is when the fix will hit Release; thru the Beta channel in Firefox 48 or later in 49 or 50?

Posted just hours ago: https://bugzilla.mozilla.org/show_bug.cgi?id=1280159#c9

Looks like the code to determine page size units on Windows OS's was re-written.

https://hg.mozilla.org/integration/autoland/rev/a4999a2b771d If you read the //comments below here - https://hg.mozilla.org/integration/autoland/rev/a4999a2b771d#l1.143 - I get the idea that Bob Owens does know "whats up" with printer code in Firefox. Kinda sorry that I have said that no one currently doing Mozilla development knows the printer code well enough to fix or modify it correctly ...

Someone who see's that problem should test it when it does hit a channel, hopefully in 48Beta6 so it can be tested ASAP.

Place the mouse pointer over the ruler (on mac), hold control key and click. This will give you a pop up window where you can select your preferred measurement.

Hopefully you are on a Mac. If you want to change your Page Set Up, from mm to inches, you need to change your default in the following (Depending on your OS):

System Preferences >  Language and  Text  >  Formats Tab  >  Measurement Units > US

or

System Preferences > International  >  Formats Tab  >  Measurement Units > US

Are you talking about printing? Page set-up is something you do in your computer OS. On Mac OSX the set-up dialog appears after clicking the print button.

You can set a custom cell size in inches in the right-hand panel of the LR print module.