The stock is the rear sight. If it fits, you can hit. If it doesn't fit, you're in trouble.
A stock which is too short makes you shoot high. Too long, low. The obvious problem is clothing--a gun you can shoot well during the warm, shirt-sleeved dove season won't work well if you're wearing a thick down jacket.
The other factors are cast-off and drop. Cast-off is the amount the stock is "bent" away from the axis of the bore, away from your cheek. This lets you easily center your eye in line with the fore-axis/sights/rib. The drop affects how high your eye is above the barrel.
A rule-of-thumb, ball-park check on stock length is to hold the grip with your finger on the trigger; arm bent 90 degrees. There should be around 1/4 inch of clearance between the butt pad and your arm.
To check cast-off and drop, close your eyes and bring the gun to a comfortable firing position. Open your eyes--you can see better, that way. Okay, if your eye lines up with the axis of the bore, and is just above the top of the barrel, you're in pretty good shape. If you're way off, it's time to go to a stock-maker.
Hope this helps, Art