So. Before felting:
Note the stitch definition and the large gaps between threads; I deliberately used a bigger needle than the yarn weight called for in hopes that the extra room would allow for extra agitation in the felting process.
Also note that amazing blaze of color in the center. I think all four skeins were the same colorway, but the first and last were definitely a different dye lot. (The first might be a different colorway; the orange is pretty distinctive. I don't know because I get fiber from M's excess stash and it's not labeled.) Anyway, also note the grey striping makes it seem wavy and wrinkled, but I assure you, it's flat.
I was so damned happy to be done with this rug that I forgot all about bagging the project and sticking a towel into the cycle to give it something to felt against. I just threw it in hot with a drop of Soak and let it go. Maybe that's the secret, although everything going through the washer for the next few days is going to sport tufts of wool.
I wish I'd remembered to measure it... unfelted, it was about 36 inches in diameter, I think. Here's a before and after with Murdercat for scale.