On the scale-debate on props (buildings are different) I cannot tell what's wrong or right. All I can tell is what I do.
As Sims aren't scaled correctly many props are like intermediaries between sims and bats.
I never used a ruler in the software. I put a car by girafe in SC4 and put one of my models next to it.
The reason is simple: I already use the girafe car props and I prefer their size so I want to have my own props to be compatible with them. To have perfered content in a compatible size is a main objective.
So that the first rule I got: Scale means nothing if the content doesn't look right compared with other content. It's not only a mathematical question but also a visual question.
And of course, f.e. some small cars I size a little up and some big trucks I size a little down - so in total they are much closer together inside the game than in real life ... but it looks well inside the game. And to me this is the important thing. Like I like girls very much. But it's not their size ore measures I care much but I get hooked by their beauty.
That's the second of my rules: beauty is all about proportions. There is no right or wrong measure but how they match each other - a very big body and a very small head may look strange but not a very big body and a very big head together. So there is no absolute measure that's right but of course there is relation - proportion which gives measure.
On street vehicles this relation is a problem. Because we have the streets quite small and the sims quite big. It's imposibble to create street props fitting both at the same time. Or they fit the streets or they fit sims.
It's obvious you have to find a compromise.
A little bit also can be done by using different street textures, without borders. But that's another project.
From my own little experience I know it's a very sad process. You have a nice model but it's to big. So you scale it down and you see a lot of details getting lost. But still too big. So you scale it down another little bit. And suddenly it looks like melted colors, blurry and ugly. That's what batting props makes a really hard challenge - to make a meaningfull shape alike to a model with a trillion of pixels but you have maybe 20 pixels at hand.
THIS – exactly this!

– is craftmanship. Making such good looking models with such reduced amount of data.
It's like carving a picture story on the surface of a walnut.

One has to regard this as highly elaborated atistry.