1. An image processing apparatus performing a character recognition process of characters written on a document, the character recognition process being performed based on image data obtained by reading the document, the image processing apparatus comprising: a character size determination section configured to (i) determine whether or not a character size of the characters in the image data to be subjected to the character recognition process is larger than a predetermined size and (ii), in a case where the character size is determined as being larger than the predetermined size, classify the character size as one of a plurality of predetermined levels, each of the predetermined levels being larger than the predetermined size; a reduction processing section configured to, in a case where the character size of the characters to be subjected to the character recognition process in the image data is classified as one of the plurality of predetermined levels by the character size determination section, reduce a region of the image data corresponding to a portion of the document and including the characters whose character size is classified as one of the plurality of predetermined levels, so that the character size becomes not more than the predetermined size, the character size being reduced by use of a corresponding one of reduction ratios which are set in advance with the respective plurality of predetermined levels, whereas another region of the image data corresponding to another portion of the document, in a case where the character size of characters in the another region to be subjected to the character recognition process in the image data is not larger than the predetermined size, is not reduced; and a recognition processing section configured to perform the character recognition process of the characters included in the image data, in the case where the character size of the characters to be subjected to the character recognition process in the image data is classified as one of the plurality of predetermined levels by the character size determination section, the recognition processing section performing the character recognition process with use of the image data in which the region of the image data including the characters whose character size is classified as one of the plurality of predetermined levels is reduced by the reduction processing section.