pulleys rated for 20kn will support rope tension of 10kn if the rope is in line with the pulley (to and fro) larger pulleys with higher kn support larger rope. Rarely will a climbing pulley fail before the rope. I'd use 1/2" static for a raft and 3/8 for a kayak or ducky, unless dyneema rope, in that case it is a smaller diameter cord for the same rating. This std rope diameter will govern the size of the pulley
Hey, if you want me to take a dump in a box and mark it "guaranteed", I will. I got spare time.