Tongue and Groove Ceiling question

Hello, I am new to this forum and had a question regarding T&G paneling for a ceiling. Hopefully I am posting this in the best spot. I am looking to put up T&G on my kitchen ceiling. I more than likely will be going with a 1x6 but was also considering a 1x4 but am worried it might look small. The room is about 18'x11'. My main question is regarding the length of boards. Am I risking warping by going with full length boards? 18's or 20's? I would assume it will obviously be a little harder to work with but like the look (and simplicity) of not having joints. I had read somewhere a while back that the longer lengths could be more susceptible to warping, is this true? Any help is greatly appreciated!