In our cars, we want it all: low emissions, fuel economy, great performance, high levels of comfort, safety, and affordability. Among those half-dozen parameters—not even mentioning that it should look good, too–are inherent conflicts.

Make it comfortable, meaning roomy to most people, and the vehicle typically grows bigger, becomes less maneuverable, and burns more fuel. Make it smaller, and some people fear that you're tampering with safety. Add technology to improve performance, economy, and safety at the same time, and the price goes through the roof, meaning, it seems, the rich get the safety.

Right now the biggest issue for new cars is economy and emissions. A bigger vehicle burns more fuel: SUVversus minivan versus sedan. Technology such as displacement-on-demand engines (which idles half the cylinders) helps to improve economy. So does the use of lightweight materials: plastic fenders, aluminum hoods, aluminum suspension housings, and even carbon-fiber roofs, trunks, and hood panels in very high-end vehicles. Replacing a steel seat pan with a plastic component saves weight and has minimal impact on cost. Putting carbon fiber on the roof sends the costs through the roof. Building aluminum shock towers (rather than steel ones) adds a bit of upfront cost but also raises collision-repair costs.

We need lightweight materials to counteract the weight of airbags, side impact beams, bigger wheels and tires, electric-adjuster seats and windows, and plusher interiors. A generation ago, the most awesome affordable sport sedan of its time— the Datsun 510—weighed 2,050 pounds. Today, a Honda Civic checks in at 2,700 pounds (and it's faster, more fuel efficient, cleaner, safer, handles better, and has a better audio system.)

What has a lot of people worried is carbon dioxide, and whether this seemingly innocuous atmospheric gas will force all cars to be smaller and possibly less safe. The Supreme Court recently told the EPA that it can't duck the issue of regulating carbon dioxide emissions. CO2 is what we exhale, and what plants inhale. Cars produce both carbon monoxide, CO, which can kill you, and CO2, which doesn't support life when the concentrations grow too high. Cars produce CO2 in direct proportion to how much fuel they consume. For years, we didn't think about CO2 emissions as polluting; now, CO2 is believed to be a contributing factor to the global warming trend that may or may not be happening (but one that seems a lot more likely to be real now than it did a decade ago).

Here's why the CO2 debate matters: The federal government sets both pollution and fuel-economy standards. California, because of its special pollution issues, can set tougher pollution standards than the rest of the country, and other states if they choose can follow California rules. New York, Massachusetts, Maine, and Vermont use California rules now. "Pollutants" means nitrous oxides, particulates, and hydrocarbons. But if CO2 is classified as a pollutant, then California also can set, indirectly but precisely, fuel-economy standards, because CO2 emissions are directly related to how much fuel you burn: A 20-mpg car produces 50 percent more CO2 than a 30-mpg car. Critics say California's government is itching to get us out of big cars and into smaller ones. (Good luck towing a boat up to Big Bear Lake with your Prius.)

And that has led to suggestions that smaller cars are less safe than bigger cars. The Insurance Institute for Highway Safety makes this argument from time to time. U.S. autoworkers who don't want to lose their jobs building big barges say it. More stridently, the Competitive Enterprise Institute makes the claim. Everything else being equal, a bigger vehicle is safer than a smaller vehicle, especially when it's running into a smaller vehicle.

But things aren't equal. Smaller cars are more maneuverable, so they may avoid some accidents (a relative handful). They're less likely to bash another car when parking in tight spaces; parking lot accidents don't cause personal injuries, but they do cause property damage accidents. Big SUVs are tippier since they're so tall, especially if you have an older model without stability control (although to be fair, so are smaller SUVs). Aside: Don't buy a new or used SUV without stability control. Just walk away.

Anyway, smaller cars have to meet the same crash standards as bigger cars. Do you think you're risking your life getting into a small Volvo? Considering all the other things to worry about, size doesn't matter.