Since I have no way of knowing what your WPF application is, and you didn't mention a printer model, or even a print driver, or an Operating system, I am left with assuming Windows, guestimating a Xerox printer of the office variety.
If the WPF application has a paper setting it probably doesn't get it from the driver, but can over-ride it, it may also have tray calls, like in Word. These settings can, <Word absolutely does> over-ride anything within a print driver.
And since WPF application do not necessarily use a print driver, or possibly use one but don't allow you to modify the settings, it may be that you need to install another copy of the printer and default that driver to tabloid, and print to that printer/driver when wanting tabloid.
Beyond that, unless you can supply access to the application in question, you would likely need to deal with whomever made the application in question since we have to assume other application are working fine to print tabloid.
