Keyword: PATIENCE!
Simple logic: you choose the length of the vigreux column on a few data:
1. What are you trying to reflux, a high or low boiling substance.
2. Is your column insulated or not, and is it needed at all. High=yes , Low boiling=not
2a. Choose the best insulation material for high boiling stuff, the better the insulation, the more preciser your reflux separation in f.ex. a 1 to 5 Celsius reflux-traject.
3. If you have f.ex. 3 vigreux columns, 10, 20 and 40 cm, does the 40 cm allow you to receive sufficient flowrate in your receiving flask? If yes, use that one, if no, step down untill you have found the right one.
If you want to know really what's needed for calculating these things, which you can easily do by trial and error, then you have to read sticky threads and UTFSE.
And buy an Oldershaw column + the rest. LT/
WISDOMwillWIN