There is no magic number. Everyone is unique. I will tell you this, "most" guys don't exactly tell the truth about size. As for the hurt... You're just going to have to tell him to TRY not to go all the way in. Sex is supposed to be enjoyable.
Average sizes vary depending on ethnicity (actually quite a bit) but for the general male population average size is somewhere around 5 inches. Large is usually considered 6.5+. Also, I don't know if I'm remembering right, but I recently read the world record is just over 13 inches.
For sex with a larger-sized man, I suggest playing around with different positions. Some of them will be much more comfortable (and easier to handle) than others.