Ankylosing spondylitis (AS) may result in a loss of spinal mobility. Therefore, evaluation of spinal mobility is an essential part of follow-up of patients with AS. The tragus-to-wall distance (TWD) is one of the four spinal mobility assessment parameters of the Bath Ankylosing Spondylitis Metrology Index, an index used in the follow-up of patients with AS. The aim of this study was to investigate the effect of body shape on TWD. In total, 465 subjects aged 18 to 65 years with no disorders affecting the spine or any joint or disturbing the normal anatomical posture were included in this study. Their weight, height, hip and chest circumferences, anterior superior iliac crest-to-wall distance (ASIS-WD), and TWD were measured. Correlations between the parameters were calculated. TWD was positively correlated with weight (p < 0.001, r = 0.389), height (p < 0.001, r = 0.174), hip circumference (p < 0.001, r = 0.296), chest circumference (p < 0.001, r = 0.291), and ASIS-WD (p < 0.001, r = 0.292). Weight gain or loss may lead to incorrect interpretations during the patient's follow-up. Comparative studies with unevenly distributed groups in terms of weight and height may also misdirect the TWD results.