The crop-waste is available in abundant to generate energy. The energy conversion process involve production of biofuel from crop-waste. This paper considers the conversion of rice straw to energy. The conversion involve the preprocessing of rice straw which includes drying and its conversion to powder form. This paper proposes the microwave assisted drying process for rice straw. The single strand of rice straw is considered in the paper. The Multiphysics simulation is performed to understand the heat generation and heat-transfer rate with variation in the size of rice straw. The proposed process involves the cutting of rice straw to specific sizes. The rice straw size plays an important role in absorbing microwave energy. The heating pattern depends on the mode of transverse electric field interacting with specified size of rice straw. The different modes getting excited in the rice straw depends on its size. It is observed in this paper that the large size of rice straw help in achieving multiple modes in a single strand of rice straw. Hence, it results in optimized microwave process to achieve the uniform heating in rice straw. The cutting of rice straw before microwave processing requires the rice straw to size between 8 cm to 12 cm.