@tspitzer , that's the hard part, agreed. Finding an image that the algorithms of a large format digital printer can 'blow up' effectively and look acceptable viewed 'close up'. An analogy is the highway billboard images; looks great at 50 yards.
Pixelated into 'colored dots' standing next to it.
This is where 'depth of field' in the original image becomes real important. The best images are those where as much as possible is in focus without much difference between foreground and background subject matter....
I used to work in commercial graphics and signage when the tech for that printing began. It's amazing what it can do of late, but again it can only do so much with a given image.
There are 'picture to poster' vendors online that you could use to print a paper 'sample' prior to committing to the final print for a reasonable cost.
Black & white/grayscale images (if that's of your taste) can sometimes 'view' better than full color. Simple imagery works better for 'near field' viewing, to borrow an audio analogy....
Anyway...good luck. Post a pic of the results... ;)