According to the Multi-Agency Radiation Survey and Site Investigation Manual, a site characterization and a final status survey are required to determine whether a potentially contaminated site satisfies the release criteria. Before conducting sampling at a site, direct measurements with a portable instrument to identify hotspot areas can provide an outline of the sampling plan to facilitate accurate analysis. As the instrument, a phosphor sandwich (phoswich) detector has the advantage of being able to detect alpha and beta rays simultaneously. In this study, large-area phoswich detectors with various probe sizes were fabricated, optimized, and calibrated to investigate variations in the minimum detectable concentration (MDC). To determine the optimal thickness of the light guide, commercially available optical design software Light Tools was used to simulate light transport. Each phoswich detector was coupled to an optimal light guide to achieve the highest instrument efficiency. The proposed instrument was calibrated using Am-241, Sr-90/Y, Tc-99, and C-14 radioactive sources, and experimental data were employed in the MDC calculation. Although it is well known that a large probe size improves the overall performance, the results indicated that probe sizes greater than 20 x 20 cm do not enhance instrument performance.