How big? Actually, it's like a flexible, clear rubber/silcone material. About 2 inches exterior diameter. Looks like it would be for plastic plumbing. Thinking about it, it's possible that that's exactly what it is; maybe I put it under the sink just in reserve (I've replaced some pipes in the last couple years) and after grabbing some plastic bags or something, it got dragged out. That is good, your water lines would be much smaller. At the joint between the pipe coming down from the sink drain to where it connects to the trap there is often a little beveled plastic gasket that you screw a cap over to make a tight seal. Maybe that came loose, might want to check those connections. Run some water in the sink and check for leak around the drain. Otherwise just ignore and assume everything is fine