A laboratory study was conducted to determine the effect of display font size on muscle activity, using surface electromyography (SEMG) and Rapid Upper Limb Assessment (RULA). The study showed that volunteers performing the experiment on small font size increased left hand flexor and neck extensor muscles activity. There was also a significant difference in RULA grand scores based on posture angle measures between the three tasks that subjects performed. In addition, there was an effect of time on upper body posture and subjective measures. There was no impact of font size on the dominant hand deltoid and trapezius, or left and right hand extensor muscles. In general, the results suggest that software developers should be sensitive to font size while designing software. The results also suggest that computer users should have better knowledge regarding the benefits of increasing the font size on their displays.