over 6 ft more than likley, probably 6'11" or so, I haven't seen any height specifics yet. We'll know his exact height eventually.
Yammy would be alot harder to answer because he grows as he conserves his energy and consumes more and more, and he seems to grow like the Hulk when angered. We know his release must be atleast 50 meters tall though, just like Godzilla.
Try waiting for the next data book, or next set of character profiles in the manga to come out; that should give us some clearer answers.