Shatapatha Brahmana 1.2.2.10

1.2.2.10

aśvaśaphamātraṃ kuryādityu haika āhuḥ | kastadveda yāvānaśvaśapho yāvantameva svayam manasā na satrā pṛthum manyetaivaṃ kuryāt

And some now say: 'He should make it of the size of a horse's hoof!' But who knows how large is a horse's hoof? Let him make it of such a size as in his own mind he does not think would be too broad.