What size rug would you go for based on this image?

[Benjmais]

What size rug should I get based on this? I have 1 corner sofa (sofa 1) and a 3 seater (sofa 2).

would a floating rug look silly?
Should I get a rug that sits underneath the sofa legs?

No-one can answer this sensibly. Go and get a blanket or two, or a couple of duvets, or a whole pile of paper taped together, and mockup you options based on the rug sizes available.

However, if it is carpet on your floor, the bits where you walk and where you sit are going to get mucky. I'd you'd do well to keep them covered with a rug that you can change our every so often.

All the front legs of the sofas should be on the rug. Maybe think about a bespoke coir or sisal rug that can be cut precisely to your specification with a cotton edging. Or a carpet that’s cut to size and the edges whipped.