Here's a potential - but only 688sqm - can u do duplex on that?
http://www.realestate.com.au/property-house-wa-embleton-115820811
You can do a duplex on 240sqm if the zoning is R80. You need to ascertain what the zoning is for the blocks you are looking at then work out what the minimum and average block sizes for that zoning.
You need to read http://planning.wa.gov.au/dop_pub_pdf/State_Planning_Policy_3_1-Residential_Design_Codes_Print.pdf
Especially page p58 which has all the sizes on it.