Quite is a little unusual in this sentence. "You are so thin that you can slip through bars" would be better.

If the sentence were negative, you could use 'quite': "You are not quite so thin that you could slip through bars." Even so, it sounds a little strange and very dated, like something people would have used in the 1800s. However, for reasons I can't explain, you need to use an adverb related to "truthfulness" in the positive sentence: "You are really so thin that you could slip through bars." "You are truly so thin that you could slip through bars." Notice that I changed "can" to "could" because you are talking about a hyperbolic possibility (unless the person really was so thin that they could slip through bars). Because it's not literally possible, you should use 'could' instead of 'can'.