The purpose of this investigation was to determine size differences between affected and unaffected upper extremities in patients with brachial plexus birth palsy (BPBP). Forty-eight patients with BPBP underwent measurements of the bilateral upper extremities. Average age at the time of evaluation was 47 months. In addition, patients or families were asked “How important is the difference in arm size and appearance to you?” Active motion was assessed using the modified Mallet classification, Toronto Test Score, and Hospital for Sick Children Active Movement Scale. Correlation between ratios of affected to unaffected limb lengths and girths and measures of active motion were assessed using Spearman’s rank correlation coefficient. Upper arm, forearm, and hand lengths of the affected limbs were, on average, 95%, 94%, and 97% of the contralateral unaffected side, respectively. Upper arm girth, forearm girth, and hand width were, on average, 97%, 98%, and 95% of the contralateral side, respectively. All differences achieved statistical significance (p < 0.01). Furthermore, over 37% of patients or families reported that limb differences were “very” or “extremely important” to them. No statistically significant correlation between age and limb length discrepancy was noted. Furthermore, there were no correlations between upper limb discrepancies and measures of active motion in individual patients. Patients with BPBP and persistent neurological deficits may expect the affected upper extremity to be on average approximately 95% the length and girth of the contralateral limb. These differences do not correlate with patient age or clinical measurements of active movement.