Purpose <p>Stimulus size and cognitive demand have previously been used to elicit changes in accommodation and pupil size, but their separate effects remain unclear. This study assessed how target size (6/15, 6/30) and cognitive demand (passive or active viewing) impact accommodation and pupil size.</p> Methods <p>Twenty-three children (mean age = 8.3±3.1 years) and 21 adults (mean age = 22.2±3.9 years) with age-normal visual acuity viewed 6/15 or 6/30 sized numbers monocularly. In passive conditions, participants were asked to simply look at the numbers, whereas in active conditions, participants silently added sets of numbers. Children who were too young to add answered questions about the numbers. Accommodation and pupil size were measured continuously at 50 Hz using the PowerRef 3. A repeated-measures analysis of variance with factors of age group (children, adults), cognition (passive, active) and stimulus size (6/15, 6/30) was performed on accommodative measurements and pupil size.</p> Results <p>Accommodative measurements were significantly greater in active than passive conditions (<i>p</i> = 0.005), but this effect was only observed in children (<i>p</i> &lt; 0.001). Pupil size was significantly larger during active than passive viewing conditions across both age groups (<i>p</i> &lt; 0.001). Stimulus size did not modulate accommodation or pupil size significantly (<i>p</i> &gt; 0.05).</p> Conclusion <p>Cognitive demand, but not stimulus size, increased accommodation in children in the present study. These findings suggest that task instruction can significantly impact accommodation, but, relative to larger targets, smaller targets may not be associated with greater accommodative measurements in clinical or research settings.</p>