Flocculent basalt fiber (FBF), a natural fiber characterized by high strength, excellent toughness, and environmental friendliness, is an ideal additive for enhancing the road performance of recycled asphalt mixtures. However, existing research on FBF-reinforced recycled asphalt mixtures has largely been limited to single-factor analyses of FBF content, neglecting the synergistic effects of FBF size characteristics (diameter and length) and content. This critical gap restricts the accurate optimization of FBF parameters and the reliable application of FBF in recycled asphalt mixtures. Hence, this study investigates the combined effects of FBF diameter, length, and content on the optimal asphalt–aggregate ratio, mechanical properties, high-temperature rutting resistance, low-temperature cracking resistance, and water stability of SMA-13 recycled asphalt mixtures. A random forest approach is adopted to quantify the relative importance of FBF diameter, length, and content on the optimal asphalt–aggregate ratio and various road performance indexes. The results show that the optimal asphalt–aggregate ratio and road performance indexes increase significantly with increasing FBF content and length but decrease with increasing FBF diameter, with minimal variation in replicate tests. However, when the fiber content surpasses 0.4%, a deterioration in performance occurs. Fiber content has the most significant impact on the optimal asphalt–aggregate ratio and overall road performance, followed by diameter and then length. The optimal fiber content is identified as 0.4% for fibers with a diameter of 6 µm (regardless of fiber length in this study) and 0.3% for fibers with a diameter of 3 µm and a length of 4 mm. These findings provide precise parameter guidance for engineering applications of FBF in SMA-13 recycled asphalt mixtures, thereby promoting the sustainable utilization of recycled materials.