1. An image size conversion apparatus comprising: an area detector which detects an interested area comprising a certain image distinguished from a general image, from an input image to be displayed on a screen comprising the interested area, and a remaining area comprising the general image; a scale ratio adjustor which adjusts a scale ratio for the input image so that a first scale ratio is applied to the interested area, and a second scale ratio, which is different from the first ratio, is applied to the remaining area; and a scaler which performs scaling for the input image according to the first and second scale ratios, wherein the first scale ratio is constant within the interested area regardless of a location of the interested area on the screen, and the second scale ratio changes within the remaining area.