Accurate characterization of transmittance and haze of transparent composites, such as transparent wood (TW), is challenging due to strong light-scattering effects. Reported TW data in the literature, particularly haze values, show substantial variability, which can be largely attributed to experimental setup parameters originally developed for transparent plastics rather than for strongly scattering composites. Here, we analyze the sensitivity of integrating sphere-based characterization methods (according to ASTM D1003) to key experimental parameters and propose measurement conditions tailored for TW. We find that, for transmittance measurements, a large input port size (e.g., 19 mm) of an integrating sphere (150 mm in diameter) is required for collecting all the transmitted photons of a 15 mm thick TW sample. Using a small input port alone can lead to a transmittance underestimation of up to 25% (from 48% to 37% at 550 nm). We also show that a smaller sphere size (50 mm in diameter) with a fixed input port (10 mm) can erroneously yield a transmittance that is about half of the expected value. For haze measurements, inappropriate choices of experimental parameters, such as the output port size, may introduce significant systematic bias, resulting in substantially underestimated haze values (e.g., 86% instead of 96% for a 15 mm thick TW sample). The proposed integrating sphere setup with adjustable ports provides a robust and reliable approach for the optical characterization of highly scattering yet transparent materials.