I use a Mac Pro (late 2013 model) to work on Lightroom 5. It has two monitors of different sizes. I prefer that LR 5 uses the bigger monitor as the Monitor 1. However, my LR5 current default uses the smaller monitor as the Monitor 1. Where can I change the setting so that LR5 uses the other monitor as the default Monitor 1? Thanks.
The primary monitor is chosen based on the setup in your system's System preferences->Displays panel. The monitor with the main menu bar is the one that Lightroom normally shows up on with its main display. If you switch those around Lightroom should switch too. You can also simply drag the Lightroom window over to the other display. To do this, make sure Lightroom is in windowed mode (Window->Screen Mode->Normal). Just drag the window over to the big monitor. This should make that monitor the main for Lightroom.