The objective of this study was to determine the effects of pen size on stress responses (during loading and unloading) and transport losses at the packing plant. This study took place between July and August. Twenty-six loads of split sex market weight pigs (n = 4,522) from three conventional grow-finish sites were used in a randomized complete block design. Each site had two rooms with both treatment groups represented in each room. The small pen (SP) treatment had 36 pigs/pen (0.59m²*pig^-1). The large pen treatment (LP) had 324 pigs/pen (0.59m*pig^-1). Both pen size treatments were sorted from pen mates at the time of marketing. Pigs were moved in groups of four to six using sort boards and electric prods, when necessary. Treatments were randomly assigned to a trailer deck (~0.42m²*pig^-1). Straight deck trailers were used and pigs were transported ~1 h to a commercial harvest facility. During loading and unloading, the number of pigs displaying open mouth breathing (OMB), skin discoloration (SD), and muscle tremors (MT) were recorded. At the plant, dead and non-ambulatory pigs were recorded during unloading, and total losses were defined as the sum of dead and non-ambulatory pigs at the plant. Data was analyzed using Proc Glimmix of SAS. Statistical analysis could not be run on the incidence of muscle tremors or non-ambulatory pigs at loading or injured and deads on arrival (DOA) at the harvest facility because there were too many zeros in the dataset. Incidence of MT was 0.04% SP and 0.00% LP and there were no non-ambulatory pigs at loading from either treatment. Incidence of injured pigs was 0.00% SP and 0.04% LP. There were no DOA’s in either treatment. SP pigs had lower OMB (P = 0.0015) and SD (P = 0.0120) during loading compared to LP pigs. At unloading SP displayed higher (P < 0.0001) SD than LP. No (P > 0.05) differences existed between treatments for OMB, MT, fatigued, total non-ambulatory, or total losses existed. In conclusion, pen size did not impact the incidence of transport losses.