I would say to go for a large rug, so it will match the sofa in scale. I personally like wool rugs because they’re lush and really easy to clean! Since this will likely be a high-traffic area of your home, that is always a nice touch!
If you’re drawn to blue tones, go blue! Then pull in the rug by placing some blue-toned pillows on the sofa! They don’t need to be the same blue to look super cute!