How do I find PageImagableArea for other papersizes

Im trying to fix a scaling issue when our application prints on other papersizes then A4. There is an existing method called GetPrintableArea in our code, but it returns the same values regardless of what mediaSize is:

public Size GetPrintableArea(PageMediaSize mediaSize)
 {
  _printQueue.CurrentJobSettings.CurrentPrintTicket.PageMediaSize = mediaSize;
  var capabilities = _printQueue.GetPrintCapabilities(_printQueue.CurrentJobSettings.CurrentPrintTicket);
  return new Size(capabilities.PageImageableArea.ExtentWidth, capabilities.PageImageableArea.ExtentHeight);
 }

capabilities.PageImageableArea W/H is always set to the same values (761, 1090). And as far as I can see this is the selected printers imageable area for A4 (in 1/96" units). How do I get the printers PageImageableArea for other papersizes like for example A3?