Mechanical ventilation (MV) and extracorporeal membrane oxygenation (ECMO) are the only viable treatment options for lung failure patients at the end-stage, including acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD). These treatments, however, are associated with high morbidity and mortality because of long wait times for lung transplant. Contemporary clinical literature has shown ambulation improves post-transplant outcomes in lung failure patients. Given this, we are developing the Pittsburgh Ambulatory Assist Lung (PAAL), a truly wearable artificial lung that allows for ambulation. In this study, we targeted 180 ml/min oxygenation and determined the form factor for a hollow fiber membrane (HFM) bundle for the PAAL. Based on a previously published mass transfer correlation, we modeled oxygenation efficiency as a function of fiber bundle diameter. Three benchmark fiber bundles were fabricated to validate the model through in vitro blood gas exchange at blood flow rates from 1 to 4 L/min according to ASTM standards. We used the model to determine a final design, which was characterized in vitro through a gas exchange as well as a hemolysis study at 3.5 L/min. The percent difference between model predictions and experiment for the benchmark bundles ranged from 3% to 17.5% at the flow rates tested. Using the model, we predicted a 1.75 in diameter bundle with 0.65 m surface area would produce 180 ml/min at 3.5 L/min blood flow rate. The oxygenation efficiency was 278 ml/min/m and the Normalized Index of Hemolysis (NIH) was less than 0.05 g/100 L. Future work involves integrating this bundle into the PAAL for which an experimental prototype is under development in our laboratory.