This is a stance first discussed in the Agni Purāna on the subject of Dhanurveda (archery).
The best description (and really only, outside of the Agni Purāna itself) that I've been able to find is:
Keep your knees thirty six inches apart and stand in the shape of a haṃsapaṅkti and this position is called Maṇḍala.
Note that the units of distance are suspect (there are conversion issues).
The Agni Purāna itself has to say:
It is known as maṇḍala if the two knees look like a row of ham̀sas and (the feet are) four vitastis apart
Probably key to understanding this passage is the word (sanskrit/prakrit) "haṃsa" which appears in both. Wikitionary suggests that this is some sort of aquatic bird, but offers a list ranging from a swan to goose to flamingo. There are also other definitions for the word that seem irrelevant (a mystical text, a man with supernatural qualities, etc).
I've found many other resources that suggest that 4 vistastis would be roughly equal to 80cm or 30 inches, probably putting the 36 inches at the far end of a range for this position. This seems to be fairly extreme, at my height it's a difficult pose and seems to have limited utility for archery especially for bows as tall as those which Indians favored.
Does anyone have any insight into just what this position looked like or how it would be used in the context of archery?
Please also consider any imagery/descriptions from Hindu religious iconography or dance, as these have been useful in helping me understand other stances described within the Agni Purāna.
New information: This page (in the 2.142.152 section) that the hamsa/bird is the goose. And that the stance itself has the knees like a "flock of geese" instead of "rows" of them. This give me a better idea of what is intended, but I still find it insufficient.