Basalt Fiber-Reinforced Polymer (BFRP) bars are increasingly recognized for their high tensile strength, lightweight characteristics, and environmental advantages over conventional steel reinforcement. BFRP bars are four times stronger and 3.5 times lighter than steel, making them ideal for concrete structures in harsh environments. This study investigates the mechanical and durability properties of BFRP bars, emphasizing the impact of bar size. Bars with nominal diameters of 12.7 mm, 15.9 mm, and 19 mm were tested under alkaline conditions for 6 months at 60 °C and subjected to 100 freeze–thaw cycles (−20 °C to 23 °C). Also, the study explored the structural performance of concrete bridge decks reinforced with BFRP bars, which are vital for resisting deterioration from deicing salts and severe weather. Tests were conducted under static loading on four single-span slabs (3 m × 1.2 m × 20 cm) and two continuous two-span decks (5.5 m × 3 m × 20 cm). Larger bars demonstrated excellent tensile properties and moderate bonding with concrete. Conditioned bars, particularly the No. 6 bar, retained significant tensile strength and resilience to freeze–thaw cycles. Importantly, durability tests showed no effect on the modulus of elasticity. A predictive model for long-term tensile strength was developed. In addition, results highlighted the effects of bar size, spacing, and continuity on flexural behavior, including deflection, crack width, and failure modes. A new empirical model was proposed to evaluate flexural-shear strength, underscoring BFRP’s durability and efficiency in bridge deck applications.