how does the poaching eggs recipe scale for different sizes of egg?
As stated above in various comments it is a complicated question. I do have some info.

First read khymos I think that the formula they have answers the question :-)
They have a answer in a graph but it is only displayed for a very small range of chicken eggs.

This page in Norwegian gives you a online tool to manipulate the size of the egg and get the cooking time, but it only handles 12 to 17 cm circumference, translation here for the terms here if you need it