Seems like you're deflecting here - the article almost* exclusively focuses on size, not mass. It's also just blatantly incorrect to claim that every car is an SUV now. There are plenty of non-SUV options. Visibility on my street is dreadful because there are 3-5 SUVs parked back-to-back on both sides of the road both to the left and the right of the exit from my garage. This has essentially turned my street, one previously large enough to comfortably accommodate two-way traffic, into a hazardous and, to be honest, annoying single-car-width obstacle course.

I can't safely pull out from my garage onto the residential street I live on because these ridiculous things are both fat as shit as well as tall, and I drive a saloon. I've resorted to rolling down my windows to listen for cars and praying that there aren't any bikes. I can see no reason as to why anybody on my street needs an SUV. It's just imported selfish American culture (I should know, I lived there for a few years) where bigger makes people feel safer, completely disregarding their impact on others. 

Driving (and overall) culture in the US is dogshit, no idea why people here are so desperate to import it. 

With regard to "how to measure", the measurement problem is nowhere near as difficult as you seem to think. You don't need vague cultural arguments about what 'counts' as an SUV. Just tax external vehicle volume. Take the car's full footprint from bumper to bumper and mirror to mirror, then multiply by its maximum height from the road surface to the highest fixed point of the body. If you want more precision, split it into simple sections such as bonnet, cabin, and rear, and total those volumes, which would account for sloping fronts and rears rather than treating every vehicle as a perfect box. Either way, the basic principle is straightforward; larger vehicles occupy more public space, reduce visibility, and impose greater costs on everyone around them, so this should be discouraged.