to smallest
It's unlikely that such a rule is mentioned in the reference, as there is a simple rule (although it's a bit lengthy to explain :D) regarding the turn order.
The stack with the highest speed goes first. In case of equal speed, the following takes precedence:
a) First, the attacking stack.
b) Then, the stack that is positioned higher in the initial formation (before repositioning using the secondary skill "Tactics").

This same sequence is maintained when moving on to subsequent rounds...