I assume by saying lightweight and hack classes at county level that you are in the UK?
Hacks are generally TBs with bone. Two categories, Large, which must not exceed 15.3 hands and small, not to exceed 15 hands. These horses must have a lot of presence, move smoothly and easily and have manners.
The lightweight hunter is again mostly TB or 7/8ths TB
Ideally about 16.1 - 16.2 with about 8 1/2 inches of bone. Should move free and easily, be very well mannered and schooled and show a good gallop. Lightweight hunters often double up as ladies hunters and are then ridden side saddle.