Well, here's my experience with ZZs.
8" lanes are very generous on straights. It gives you room to pass or zig zag, or experiment with different lines, or place obstacles
For 90° curves and even 180° cruves, 8" is still fine.
For S curves, you need more room. Though you can make it around 8" wide S curves without hitting a wall, it's really hard to do. 10" wide lanes around S curves make for better racing in my experience.
The first track I built was around 45" x 54" total size. The lanes were all 8" wide, and I had a lot of trouble with the S curve. (I've attached a pic of the layout of that track.) So, I modified the design into the middle image. There are fewer curves, but the curves are wider.
The size (45" x 54") is 4 feet by 4 and a half feet. I like the size because I can put on my dining room table or the floor, and I can prety easily move it out of the way when I want to use the dining room table for something else... like eating. I don't like the size because 54" isn't that long of a straight section. It's long enough to get top speed around the outside even with the 2 curves, but you can't hold top speed for very long. I'm probably being to fussy about that though. But I still get track envy when I see a 4' x 8' track.
For guard rails, I'd go with 3/4" or more. The cardboard pieces I'm making only have 1/2" walls and I can get a ZZ stuck over them. But, that could just be a result of the cardboard piece's modular nature. If it were a continuous rail, 1/2" might be fine.
I am working on a modular track now, because to be perfectly honest, the same old track gets boring. I'd like to be able to easily rearrange the track.
The track in the pic is quite fun for a few reasons even though it isn't modular. It has the 3 outside walls for a nice, long, speedway, and a series of sharp curves for technical driving. And, the added bonus is that you are using your whole surface to race on. If you add some gates to this design (movable walls) you can get a few varieties of tracks pretty quickly. I drew up a design with 3 gates so you could quickly change the layout of this track to an oval, and two other simple variants.
I hope this helps!