If the rug cannot hold the sofa's front legs, it is a bath mat

The rug is the ground the seating claims. When it stops short of the front legs, the sofa looks like it missed its mark and the rug looks like a sample.

As a rule, the rug should run under the front legs and extend a little past the side tables. In a tight room, all four legs on is even calmer. In no room is 'just the coffee table' a finished idea.

Drop both objects in the studio and look at the leftover border. A thin halo of floor around a well-sized rug is fine. A wide moat around a small one is the bath-mat effect.

Colour can wait. The rectangle has to be right first, or no dye will save the arrangement.