1. A computer-implemented method, comprising: at a portable electronic device having a touch screen display: displaying a plurality of key icons, each key icon having a fixed displayed size and an adjustable undisplayed hit region of dynamically adjustable size; receiving a sequence of individual touch points input by a user on the touch screen display, wherein: each touch point is determined at lift off of a contact from the touch screen display, and an image with an enlarged version of a character that will be selected as the character corresponding to an individual touch point is displayed prior to lift off of a respective contact, wherein the character image that is displayed prior to lift off is selected in accordance with the adjustable undisplayed hit regions of the displayed key icons; and after receiving each of the individual touch points: forming a user-input directed graph for the sequence of individual touch points received so far; determining a character corresponding to a last received individual touch point in accordance with the adjustable undisplayed hit regions of the displayed key icons; displaying a sequence of characters corresponding to the sequence of individual touch points, including the determined character; and updating a size of an adjustable undisplayed hit region for at least one of the plurality of the key icons in accordance with the sequence of individual touch points input by the user, wherein the updating includes changing the size of at least one adjustable undisplayed hit region.