Side bearing dimension

I am assembling a 10" f5 tube dob, but I am not certain as to which size to cut the side bearings as to achieve the speed break effect of teflon on ebony star. Could someone please advise as to the optimum diameter or formula so I can get on with this project. Thanks

If you read the section in the Kriege/Berry book about building a small tube dob, they reccomend having bearings 1.8x the tube diameter. So lets say your tube it 12" in diameter. So then 12*1.8=21.6. So I would say 21.5" bearings would work according to the book.
I'm not sure however if this only applies to their design.