This　study　explores　the　mechanical　properties　of　Glass　Fiber-Reinforced　Polymer　(GFRP),　a　high-performance　composite　material,　focusing　on　how　varying　diameters　affect　its　tensile　strength,　modulus,　and　elongation.　Experimental　data　obtained　from　three　sets　of　tensile　tests　on　10,　12,　and　25　mm　bars　helped　establish　a　stress-strain　relationship　for　GFRP　reinforcements,　considering　diameter　changes,　and　a　formula　for　calculating　the　ultimate　tensile　strength　based　on　diameter.　Utilizing　the　weakest　chain　theory　and　the　Weibull　distribution,　the　research　found　that　GFRP＇s　tensile　strength　diminished　with　increased　diameter,　while　the　elastic　modulus　behaves　oppositely.　The　analysis,　grounded　in　the　weakest　chain　theory,　identifies　the　specimen＇s　effective　volume　as　a　critical　factor　in　the　size　effect　of　GFRP　bars.　Moreover,　the　study　proves　a　significant　size　effect　on　GFRP＇s　tensile　properties,　validating　the　theory＇s　application　in　predicting　the　strength　of　GFRP　bars　of　varying　sizes　and　recommending　a　specimen　length　range　of　30-40　times　its　diameter　for　standardization　purposes.