Thank You letter doesn't need to be much longer than two sentences. If it's some great screed over two pages saying "Thank you, thank you, thank you," then it gets a bit saccharine. Starts to cloy. Like having to swallow a whole mouthful of Old Nurse's Fudge.
One thing is for sure - saying thank you is a nice thing to do. It makes you feel good, and makes the receiver feel valued. We may not subscribe to the hand written parchments of old, with ruler straight lines and wafty words of gratitude penned from inky quills, but we do still subscribe to basics of liking to give and liking to be thanked.