That assumes uniform marked parking spots. A lot of cities make heavy use of roadside parking where your car takes up as much space as it's long plus what's needed to be able to pull out. With roadside parking there's a big difference between a 3 meter ultracompact and a 5 meter SUV. Likewise, wider cars might take up space on the road and/or the sidewalk.

Even with marked parking spots long cars may exceed the length of a parking spot that was perfectly adequate when it was designed. I have something like that near me. There are parking spots perpendicular to a road and long cars routinely obstruct the sidewalk because otherwise they'd be obstructing the road.

I'm all in favor of having way fewer cars around in general but I also want the cars that are there to coexist better with everyone else.

No, it also matters for some admittedly unfortunate street layouts. E.g. here in Oslo we've had some one-way streets with parking on both sides, where we've had to remove parking on one side to allow buses and emergency services continued access in the age of SUVs.

Anything that has some sort of geometric constraint gets worse with big cars, and having an extra disincentive on huge cars is good.

edit: Car size creep will also eventually find its way into parking requirements, and parking norms, terrible as they are, require a certain amount of parking spots, not area dedicated to parking, which will make construction even more expensive.

Most parking garages do have compact car spaces, but otherwise street parking and parking lots are standard sizes.

Although for areas with open street parking (no metered spaces) the smaller cars allow more people to park

Admittedly, those are already handled by something else where I live: A regular class B license only lets you drive vehicles with a max weight of up to 3.5 metric tons; everything beyond that requires some kind of appeal l special license and those cost a couple thousand bucks a pop and have additional strings attached. Not something you get casually.

So I would handle commercial vehicles like freight trucks, buses, tractors etc. differently from regular cars. The use case there is usually that someone has a specific professional need for such a heavy vehicle and not that they want to drive their kids to soccer practice in an Actros.

If your country doesn't have truck licenses and random jackasses do drive around in semi trucks for fun you can just go with a tiered approach. I'll use the European definitions of vehicle classes here; adapt as necessary.

Commercial vehicles get taxed by maximum weight. That includes everything designed to pull a semi-trailer, tractors, self-driving farming equipment, everything capable of seating more than eight people, and everything with a max load above 3.5 tons.

Personal vehicles (ie. everything that don't match the above criteria) get taxed by empty weight.

So if someone decides they want to drive around in a Peterbilt with no trailer attached they still get taxed as if they were hauling heavy loads all day because that's what semi trucks usually do. But someone in a regular car gets taxed as if their car was mostly empty because that's usually the case there.