Photos of extractors are a start on collector length. First choose whether you're going Tri-Y or 4 into 1. Tri-y collects 1 and 4 together and 2 and 3 together before merging the two secondary pipes.

My extractors (on a A15) have primary pipes of about 400mm and the secondaries would be around the 1 meter mark. You'd probably be happy with a length around 1.5m for 4 into 1 extractors (based on observation this is about how long they are). YOu'll notice the GX exhaust also has about 1.5m before the 4 cylinder pipes merge.