Attached Link: http://openhardwaremonitor.org/

When a monitor says it's going into power save, it usually means that the video subsystem has stopped sending signals to it. I'm wondering if your PC overheated; you might want to check that the fans are working, especially the one on the video card if there is one. You could try running Open Hardware Monitor while you're rendering to keep an eye on temperatures, fan speed etc.

Rendering a larger image shouldn't require much more memory, just more time, which is another thing which makes me wonder about overheating.

That said, I have noticed it seems possible to push Firefly 'over the edge' in ways that I don't understand. One not particularly complex scene I'm working on takes an age to render in its entirety, but if I separate the characters and buildings and render them separately I have time to run two renders, composite them together and take a coffee break, and it's still quicker than rendering the whole thing.

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By the way, before some pedant takes the OP to task for claiming to 'crash' a monitor, it is possible. My day job involves investigating transmission errors in digital video paths, among other things, and I've seen that some monitors will just go to sleep if they see too many errors. You have to turn them off and back on again. ;)