Im in the market for a 20 ga semi auto for my wife. She's not verry big (5'3") and Im not sure if I should get her a youth model. How do I measure her for the LOP that will fit her best? Also if you know of an inexpensive gun you can recomend for clay shooting let me know.

The best way to do it is to go shoulder a few models and see what fits her best. There are too many variables to accurately measure LOP.

There are no variables in measuring LOP on a stock its always the front face of the trigger to the center rear of the butt. It is difficult to measure this on a person. The absolute best way to measure it is to go to a custom stockmaker and be fitted on a try gun. Lacking that do as MattH says. A youth model may be too short and a standard stock too long. If you only need a little added (like a different recoil pad) get a youth. If you have to make a std stock only a little shorter get that and have the stock cut down. In other words get the stock that will require the minimal modification.

Like Griz says, the least mods will be the best. Go to the Dicks in Brick on rte 70. They always seem to have youth model shotguns. Try them on for size. If it's close to fitting, then she could make do with a buttpad spacer that adds to LOP. She probably won't want a recoil pad because that can absorb enough recoil energy to make the semi-auto action unreliable. It would be fine for a pump action or break open though. If she needs much more stock length than the youth model gives her, then someone will need to cut down a full size stock and put on a grind-to-fit buttpad. A gunsmith could do that very inexpensively. If you are handy and adventurous, you could do it yourself.