How long does a Thank you letter need to be?

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I prefer the two or three sentence format. I face palm when I get a thank you email that looks like a formal business letter.

(sent from my phone)

Ditto. All you need to do is thank them for interviewing you and maybe add a personalized sentence about something that was mentioned during your interview so it doesn't totally look like a form letter. That should be it.
 
A couple of sentences is fine. Keep in mind that most of the time, interview thank you notes have zero bearing on your application. It is still common courtesy though.
 
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