Selection of the Samples with Probability Proportional to Size

It is manifested to all that sample size varies from unit to unit. It goes without saying that large units contain more apropos information than the smaller units. So if the unit size is larger then there is a greater possibility to choose sample from the large unit than smaller one. It actually means the probability of selecting a unit is positively proportional to its sizes. The selection of unit is done corresponding to choose a number at random from the totality of numbers associated. My main aim is to prefer a method of selecting units on the basis of its size.