Ah! Gotcha. Well, here's what I know:
- Everything must be linked. Standard link distance limits apply, so unless you are using non-phys scripts, there's an implied size limit of ~40m. Non-phys will let you do bigger vehicles, but I think physical vehicles tend to drive smoother.
- No more than 31 prims including passengers.
- Each passenger counts as one prim. So if you want a pilot and a co-pilot, your max is 29 prims, etc.
- Be warned that sculpted prims use spherical collision boundaries regardless of the shape of the finished sculpty. It's often best to avoid them for the body of your craft and limit them to detailing or interior features like chairs. If you use them for the body, be sure to use the "normalized" mode in the exporter to fill the collision boundaries as completely as possible.
- The whole craft rotates around your root prim, so it makes the scripting easier if you make the root prim whereever the center of gravity of the craft is. If you have extra prims, some people make a small invisible sphere to serve as a pivot point and use that as the root prim.
- Larger prims consume more "energy". "Energy" doesn't seem to directly correspond to any known physical phenomena, but somewhat mimics something between the consumption of fuel and inertia. The net result is that larger craft have a more difficult time staying aloft. Hover is most affected, but you can sometimes work around this by applying an impulse. In any case, you should consider the size of your craft depending on your desired speed\altitude.
- Round nosecones or an invisible "bump rod" are recommended. Flat-nosed craft tend to get stuck on things.
- Building interiors is pretty much just like building exteriors... no special rules here AFAIK. I sometimes use hollow prims, or surround the pilot in multiple prims if I'm make a canopy or something of the like.
I hope at least some of that is helpful.
