This study investigated the impact of the control specimen size and geometry on the thermal performance of geopolymer and Ordinary Portland Cement (OPC) concrete under high temperatures. With the increasing demand for sustainable construction materials, geopolymer concrete has emerged as a promising alternative. However, their behavior under extreme thermal conditions remains a crucial area of exploration. This study adopted a systematic approach to evaluate the influence of variations in the control specimen size and geometry on the thermal response of both geopolymer and OPC concrete. This study involved the casting and testing of control specimens, including cubes, cylinders, and prisms in M30-grade concrete. Compression tests revealed the highest strengths of 37.2 N/mm2 for 100 × 100 mm cubes, 33.12 N/mm2 for 150 × 300 mm cylinders, and 34.08 N/mm2 for 150 × 150 × 300 mm prisms. These analyses aim to reveal the intricate interactions between the size, geometry, and material composition. These findings offer valuable insights into optimizing the design and performance of geopolymer concrete, especially in comparison to traditional OPC concrete, when exposed to high temperatures. Subsequent temperature studies of these critical specimens revealed varying effects. A comparison of OPC and GPC specimens at different temperatures demonstrated significant differences. For instance, at 200 °C, OPC cubes exhibited a strength of 39.2 N/mm2 compared to GPC’s 45.65 N/mm2, with a percentage difference of 1.15 times. Similar trends were observed for cylinders and prisms at different temperatures. This study advances a more comprehensive understanding of the thermal behavior of alternative construction materials, laying the foundation for improved structural resilience and sustainability in extreme conditions.