As for the Major, it is a 27/64 (on EB site) and I believe the length is the same as for a Sierra twist ... 2.21"
The Deco BP tube length is about 3.03" ... I say "about" because I am measuring off a pen I completed a few days ago and not off the bare tube.
I was confused, at first, because I was thinking of the Jr. Harold RB and FTN which are the same as the Jr. Gent II. So if you read this post when I first wrote it that's what I was thinking. But I realized my mistake and edited.