With FTA, the bigger the reflector, the better. Larger dishes (of similar quality) will give you better resistance to poor weather problems and they will do better in "fringe" signal areas. If you have a specific need for a particular satellite and that satellite covers your location, a smaller dish may do fine but if you're in it to see what's available across the sky or use satellites that don't expressly cover your location, larger is better.
Signal strength reception is directly proportional to the area of the dish. You can use the ratio of the squares of the diameters for circular dishes to calculated the expected percentage increase/decrease in signal strength.