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Live traps are commonly used to inventory, monitor, and sample populations of small mammals. Due to the variety of available trap types, understanding differences between traps is important to minimize bias and plan future studies. Sherman traps (H. B. Sherman Trap, Inc., Tallahassee, FL, USA; hereafter Sherman traps) are a popular live trap that come in a variety of sizes. However, studies comparing the relative efficacy of different‐sized Sherman traps often focus on a single species or are limited by a lack of temporal and spatial replication, leading to contradictory or ambiguous results. Therefore, to better understand the relative efficacy of two commonly‐used sizes, we used a paired design and 10 years of trapping data to compare species richness, capture numbers, and mortality rates between small Sherman traps (5.1 × 6.4 × 16.5 cm) and large Sherman traps (7.6 × 8.9 × 22.9 cm) across 55 preserves in northeastern Illinois. Despite wide annual variation, we captured more small mammals in large traps except for the smallest taxa (Sorex spp.), the proportion of captures in large traps increased with body size, and large traps significantly reduced mortality rates for Peromyscus spp., meadow voles (Microtus pennsylvanicus), and short‐tailed shrews (Blarina brevicauda). We recommend the 7.6 × 8.9 × 22.9 cm Sherman trap for monitoring surveys in the midwestern USA when research or monitoring objectives include maximizing captures and species richness while minimizing mortality. However, we note the smaller‐sized trap may be useful to minimize bycatch when smaller species are being targeted. Finally, managers and biologists conducting short‐term or pilot studies should be wary of year effects, as we found substantial annual variation in relative capture rates between trap sizes. © 2021 The Wildlife Society. [ABSTRACT FROM AUTHOR]