Abstract: Straw biomass, regarded as a promising low-carbon alternative to coal for power generation, is susceptible to self-heating during storage, a process that may lead to spontaneous combustion and pose fire risks. In addition to moisture content, both the scale of the straw pile and the particle size of the material significantly influence its self-heating behavior. To investigate these effects, this study designed and implemented experimental setups using a 3 L foam canister and a 120 L foam container to examine the self-heating characteristics of corn straw. Two forms of straw samples were tested: powdery particles with a diameter of 2 mm and raw stalk-like pieces measuring 10–15 cm in length. Their self-heating behavior was monitored and compared with that observed in a 3 L Dewar flask. The results showed that the 120 L foam container, lined with insulating cotton, exhibited the least temperature loss and the best thermal insulation performance. The scale of biomass accumulation substantially influenced self-heating characteristics, with larger piles showing more pronounced temperature increases due to enhanced microbial activity. Both powdery and stalk-like samples displayed stronger self-heating effects in the larger insulated container, as reflected in the rate of temperature rise, peak temperature, and stabilized cooling temperature, thereby enabling more effective observation and analysis of the self-heating process. Particle size also played a critical role; larger particles corresponded to weaker self-heating effects. Compared with powdery samples, stalk-like samples—characterized by lower specific surface area and higher porosity—reached lower peak temperatures. However, their temperature declined more slowly over the detection period, resulting in higher final stable temperatures.