3 questions 3 answers
1. Yes they should. the endplates separate the air like a "plenum"
the common theme is fast-under Slow- over. the high pressure very badly wants to make its way to the low-pressure air to help balance the atmosphere. this is generally how the wind is generated. if your endplates are too short the high pressure on top of the wing will find a way to the bottom when this happens the wing will stall and stop making downforce. the endplates help prevent the air from mixing. so yes make them long, and have them over the entire chord and add a few extra inches for good measure.
2. yes the shape does make a difference. But there is no magic bullet. For our wings we offer CFD Built endplates, that means the computer software was used to shape the endplates. They had a small bump in downforce, But what it helped was efficiency not stalling in a crosswind. On the phone, everyone from the northeast seems to understand that concept personally. something about the glen. So yes it does matter but you couldn't take our CFD endplate and expect the same result on a competitor's wing. the shape of the wing plays a major role on the endplates shape. it's a frustration point to us when we hear people endlessly quoting F1's aero designs. You have a gt car, not even a formula car. stop looking at F1 and get your nose into the current gt3 offerings. know the rules and why some pieces are the way they are. After that your free to gather inspiration.
3. When in doubt go big! you're looking for that plenum effect, a tiny endplate might allow air to bleed over when a large endplate would stop it. Having too large of an endplate almost never means a loss of downforce.