Depends on the design, but I would say 6" is the absolute minimum provided the section is from a different material. If making the section from the same material without the need to pattern match between cap an body, then I use two 5" blanks. If I need to match the pattern and make the section, then 9".
For the worthless wood pens of mine that I know you have seen, the WW cast was 6.5" with a second 3" blank poured from the same mix st the same time for the section.
So, where does all this lead? I think a 10" blank is the best bet for generic usefulness - it can be used to make 2 kit pens or 1 kitless, with just a little bit extra for an "oops", and allows for a pattern matched cap/body if needed/desired.