from
http://www.deltachiconvention.org/Delegate/index.htm :
Each undergraduate chapter shall cast a number of votes based on its membership, defined as the number initiated and dues-paying members including those initiates who are not yet required to pay dues, as follows: if its membership is less than the average chapter membership, three (3) votes; if its membership is equal to or greater than the average chapter membership, four (4) votes; if its membership is equal to or greater than twice the average chapter membership less ten (10), five (5) votes.
There's info in that link on colonies and alumni chapters too: they get votes proportional to size, and all the past International Presidents in attendance form a committee that also gets two votes.