SLimlines can be turned from 1/2 square stock if you have perfect drilling capabilities. And you are turning the blank bushing to bushing. It does not leave much room for error. A 5/8 x 5/8 x 5in blank will do nicely. You have some room for error on drilling and you can play with the shape if you so desire. You will need a little extra rather than exact length of the tubes on the blank to allow for triming and squaring the blank to the tube. I have used blanks that were 7/16 square but its touchy. everything has to go exactly right. larger blanks will work but you will have more waste to remove. If I use a larger blank than is needed I often trim the blank on the band saw and set the cut offs to the side for segmenting or for use as a contrasting
center band or finial on a different pen.
Mike