The value of the futures contracts on Nifty may not be less than Rs. 2 lakhs at the time of introduction. The permitted lot size for futures contracts & options contracts shall be the same for a given underlying or such lot size as may be stipulated by the Exchange from time to time.
Check this link:- http://www.nseindia.com/products/content/derivatives/equities/cnx_nifty.htm
As Praveen has rightly put, the guideline that NSE follows is to keep the value of the contract above Rs 2lks at the time of introduction, this is again a regulatory requirement from SEBI. As and when the value of stock goes up quite a bit above Rs 2lks, the contract size is reduced to bring it back to around Rs 2lks. For example Nifty futures when first introduced was a lot size of 100, but when Nifty went to 5000+ this lot size was reduced to 50.