Ok, I'm at home now and can throw down a bit more info.
The standard stock car size is 18" x 24". You can get these for cheap $20 each, so $40 for a pair. Speedway, Summit, Jegs
If you can make that size cover your windows then you're golden. There has been A LOT of lieniancy in XRRA on window nets in my opinion. I believe the net needs to cover the window opening, and leave no margin of error for a hand or helmet to get past.
This is an example of a window net that I think is technically doing its job on paper and has passed tech to race, but if called on for use, it will fail, and you risk injury. This would not pass tech is on a stock car, roundy round, etc.
Here are mine. Maybe mine are overkill. But not only will my appendiges stay in the car, they will keep everything out. Like a big rock if you flopped onto it, and it wanted to protrude through the window. I tabbed the top bar and bolted it, the bottom is the Speedway latch mount kit.
You can kinda see mine is this pick of Bones that Nolen took. The tabs and bolts on top, and you can see a small hole at the base of my "B" pillar. That is were the bottom rod sticks into. On the front of the bottom rod is the latch mechanism.
Now I really like the type that Bigelow, Campbell, and the Shirleys use. Its a mesh style that is affixed at the bottom, and when you put it in to race you raise it and lock it in. I think its springloaded, and when you grab it, its drops out of the way. So getting out in an emergency is FAST.
Matt what one is best for only using every now and then? I dont have the answer, I'm thinking on mine I will leave the front and rear top tabs, and cut off the other three. Then drill a piece of flat stock and mount it to the top of my net so it stays up and rigid. And I wont be hitting those middle tabs with my head getting in and out..... I dont know. But I'd look at the style those guys have on their moonbuggy's They may only work on moonbuggys because there is some distance between the driver and the side in a single seat application..... I dont know.