Hello, I finally figured I'd chime in. I only have one thing to say, have fun with it! Do what looks good to you, throw the rules out the window. But in all seriousness, LEGO minifigures aren't human, they don't bend, sit, or share proportions with us. With that in mind, worry more about looks, details, and structure over scale and rules. I personally have been build a small fleet of custom 6-wide LEGO City styled vehicles and I find that the look great next to the modular buildings. However, if you are worried about scale and whatever, ignore what I just said and do what looks great for you. In the case of your parking lot, I would suggest lengthening it like Sollx said. Maybe use some 8x16 baseplates to widen the lot to fit real vehicles. As for inspiration for a consulate vehicle moc maybe use some BMW M3 or M5 with some Kia Stinger mixed in. If your looking for a pre-designed vehicle though, maybe shorten the limo from 60102 into a 4 door styled sedan or maybe use some of the 6-wide police cars from 2017 police line as inspiration for some mocs.
Have fun building and I hope I helped.