Není to správně protože ve větě začínající proč, jak,atd. slovo "je" musí být na druhém místě. (In sentences which begin with Why, How, ... the word "je" have to be on second place.)

Same sentence structure as English applies here. You wouldn't say "Why your car is small?", you would say "Why IS your car small?". But this is clearly for purely learning, considering no one would ever ask that question like that. How would one answer that anyway? "Cause I couldn't afford a bigger one?" haha

If anything, one would ask "Why do you have a small car?"...Because I prefer it; better mileage; etc.