Some of the elements instructional designers must consider when designing screens for computer based instruction (CBI) include legibility, readability, and particularly font size. The purpose of this study was to determine which size font is most effective for CBI. Participants were 163 undergraduate and graduate students enrolled in an upper-level computer course at a western university. Subjects were randomly assigned to one of four treatment groups. Each treatment group received a different font size. All subjects received a computerized version of the Nelson-Denny Reading Test Form E. The four dependent measures were reading comprehension, satisfaction, preference, and distance from computer screen. The three independent measures were font size, age, and section. Results indicated a lack of satisfaction with the assigned font sizes of 10 and 12 points and satisfaction with font sizes of 14 and 16 points. Subjects preferred the 16-point font size to other sizes. (Contains 24 references.) (AEF)