The average size Chi within standard is about 5 to 6 lbs. This size Chi is quite small, and many times referred to as a teacup size. Since there is so much misconception about Chi size and weight, many do not realize how tiny a 5 to 6 lb. dog is. Breeders will tell people that say the parents are 3 lbs. When actually they are 5 to 7 lbs. They easily pass for this weight, because they are so small. Anything under 4 lbs is by far the norm. You'll see a hundred in the 5 to 7 lb range, and very very few under 4 lbs
Now days Chi's are bigger. But actually if you look back at older pics of Chi's most were 5 to 7 lbs too.
It is very common to see 6 to 10 lb Chi's. Even at 10 lbs they are small dogs.
As Ashley mentioned, pics can be hard to judge unless you see many shots from different distances. Up close pics can make them appear larger, while far distances can make them look smaller. I've found a good way to judge is by seeing someone hold them. This will vary somewhat too depending on the size of person holding them. Without seeing several pics, it would just be a random guess.
But unless your Chi is over 10 lbs, he's probably about average.