How to determine peer institutions?
The Carnegie Foundation gives 6 types of criteria:


Undergraduate instructional program
Graduate instructional program
Enrollment profile
Undergraduate profile
Size and setting
Basic classification


There are many factors involved--for instance, geographic region and percent of full time students are factors in 5 and 4.  I don't think semester versus quarter system is too important.  See the link for descriptions of these criteria.

There also seem to be a couple of Chronicle articles about selecting peer institutions:

http://chronicle.com/article/in-selecting-peers/134228/

http://chronicle.com/article/Who-Does-Your-College-Think/134222/

however I can't access the Chronicle website right now, but did find this other article which discusses the first Chronicle article linked:

http://theairspace.net/commentary/u-s-colleges-name-their-own-peer-institutions-rank-themselves/