Have a look at the EPC for a property, at the top right hand corner of the full EPC it gives the habitable area of the property in sq metres (not including garages etc.)
That way you can determine the price per sq m / sq foot of the property!
Make notes to compare prices between properties!
All propertys have to have an EPC when they are for sale (or rent) and the estate agent should show it you free of charge.