He can use a measurement device such as a trundle wheel to scope out the length and width of the property. This should tell him the area of the property if it's a rectangle just by multiplying them together. He'll have to do some more math if it's irregularly shaped, but the rectangle area will give him a good idea of the size of his new property.
Mr. Peters should get a residential survey done. It will give him the accurate measurements of his property by determining the correct boundary lines. A survey ensures that the land he buys is correctly measured and defined for registration with local government offices.