Rule of Thumb for separatory funnel volumes
If a general rule must be given than the volume of the separatory funnel is mostly dictated by that of the liquid to be treated.

Indeed is better to extract the product by repeating several steps each using small portions of extracting solvent. This ensures a more efficient extraction based on repartition, I.e. a more quantitative recovery of the product as well as a more convenient work-up (cost-effective, less time consuming, less waste).

I am not considering difficulties due to a slow phase separation which might prevent the use of a narrow funnel but in general use the smallest that allows an easy shaking and inter mixing. (When you get to know the actual separation you might even prefer a  specific funnel shape not just that V).

Example: 200 ml to be treated with three to four 25 or 50 ml portions : 500 ml funnel.