To investigate the size and slenderness effect on the axial-compressive behavior of basalt fiber–reinforced polymer (BFRP)-confined predamaged concrete, five groups of concrete cylinders with different sizes and slenderness ratios were designed and tested. The cylinders were axially preloaded to three predamage levels, then repaired using BFRP, and reloaded. The results showed that the concrete predamage had an adverse effect on the ultimate strength and initial elastic modulus of BFRP-confined concrete. Except for the smallest specimens affected by the wall effect, the initial analysis found that the ultimate strength of BFRP-confined concrete decreased with an increase in size and slenderness ratio, and the size and slenderness effect decreased with an increase in BFRP confining pressure, while these increased with the severity of concrete predamage. However, there was no obvious size or slenderness effect on the ultimate strain of BFRP-confined concrete. Through multifactorial analysis, it was confirmed that the ultimate strength of BFRP-confined undamaged and predamaged concrete was influenced by the slenderness. Considering the effect of size, slenderness, and predamage, monotonic and cyclic models were developed for BFRP-confined concrete. Finally, a uniaxial material object was added into OpenSees to provide an effective numerical material model for theoretical analyses and engineering applications.