From the chapter's website:
http://www.zetathetachapter.homestead.com/
"Zeta Theta Chapter is the only Black Greek Letter Organization Chapter on Purdue University's campus to have a house. The DELTA HOUSE is the result of hardwork and a true commitment from Zeta Theta Chapte Alumnae and present members.
...
Presently there are 5 members of Zeta Theta Chapter who give of their time and talent to serve the Greater Lafayette and Purdue University communities."
I believe it is smaller than the other 3 NPHC groups on campus, but I think the NPHC groups are generally smaller than the NPC groups - at Purdue at least.