This study investigated the length estimation skills of high school students with mild intellectual disabilities (N = 39) by means of a paper-and-pencil assessment and interviews. The results showed that the students performed differently in different estimation situations involving size discrepancy and accessibility of to-be-estimated objects. The students tended to underestimate the lengths of daily objects. The uses of body parts, objects in convenience, mental rulers such as 1, 10 and 15 cm as reference points through unit iteration were the strategies reported by the interviewees.