An article "a" does not make any sense in this context. If you like something, you would use the definite article in case of just one pair of trousers, but...
The sentence "Lubisz wąskie spodnie?" (Do you like skinny pants?) is a very general question, so it refers to plural, not just one pair of pants or trousers.
The sentence "Lubisz te/swoje wąskie spodnie?" (Do you like these (those)/ your skinny pants?/ Do you like this (that)/your pair of pants?) may refer to many or just one pair of pants, for a change.