I'm sorry this happened to you, I've had things shouted a few times about my size, out of cars and it feels horrible at the time!
I had something happen a few days ago actually and after a run of 6 rough, long night shifts and having to get up off the last one after 3 hours sleep for a dentist appointment, I really wasn't in the mood.
The car slowed down to let me know what they thought of me being a "Fat Cunt" and so I stopped and shouted "Come on then, get out and say it to my face!" Back, he brakes and made as if to and I just stood there like 'Come on then!' and then the big, brave hard man drove off, wheel spinning.
If he was really that big, hard and clever, he'd have taken me up on the offer wouldn't he, were he so full of self belief in what he was saying.
It's made me think differently about it now, that in reality, if someone has to shout something at you while moving away, safe in their little tin box, they're a bit of a coward really, and that's not someone who's opinion I care for in all honesty.
I wouldn't recommend doing what I did, and I don't think I'd do it again because it could be dangerous, but I am actually glad I did because it's changed the way I think about it. Instead of letting it get to me and thinking I'm a lesser person in their eyes, I now think it's a bit sad and cowardly and they're a lesser person for not having the guts to even give a chance for someone to respond, because they're scared of the concequences. They're only brave enough when they know they'll get away with it.
A friend once really impressed me when she was called a fat cow a few years ago, before I put weight on and so hadn't experienced it myself - she replied by rolling her eyes and saying "God, can't you even think of an original insult? It's not like that's obvious or anything!"
He left mouthing and muttering to himself and his mates laughed. Was a great way to turn the tables.
What happened to you was awful and I'm so glad you haven't let it put you off, I too think it's assault and a shame you can't ID the car and report it, but I can imagine the shock of being dowsed in a cold milkshake was quite disorientating at the time - hope it's washed out of your hair OK.
I just don't know what's wrong with some people that they consider this type of thing funny or entertainment really.
Good luck with your continued fitness! 💐