I've worked in online learning at 3 universities in Australasia in the last 10 years.  The policies at all three have been similar, but the thinking in this space is evolving.  I'm reading 'cohort' to mean 'a teachable group'...not 'everyone who enrolls in a course. 

Briefly,  cohort size here is linked to cohort sizes in on campus teaching for administrative reasons, not pedagogical ones...but it works out ok.  A notional cohort size is 25-30.  For on campus teaching at UniSA, 25-30 is 'a tutorial group' and that's how many students are scheduled to be in a given tutorial room for their on campus sessions at any given time.  Online, there is an administrative equivalent where students are grouped in cohorts of 25-30 and that is done automatically in the link between the student management system and our online learning platform.  As students enroll, they are allocated to tutorial groups up to the max number (e.g. 30) and then a new group is started.  The system is not perfect...its based on a notional enrollment number which implies a certain number of tutorial groups (total enrollment/30 students per group)...and so there is some last minute tweaking just as teaching starts to combine or divide groups as needed.

Beyond that, its not policy driven.  A variety of grouping strategies are used in the case where a) the 'tutorial' model has not been applied and there is a large, ungrouped cohort of online students or b) the teaching staff want to create 'groups within the group'.  In the former case, we usually aim to create de facto tutorial groups of 25.  In the latter case, anything from pairs to small groups of 4 to project groups of 6-8 is used.  One of my jobs as an academic developer is to work with teaching staff and think through the grouping strategy to align it with the intended learning activity in the course.

we dont any policy about it