To design MRI/ultrasound (US) dual modality imaging probes with optimized size for prostate cancer imaging by targeting prostate-specific membrane antigen (PSMA). The PSMA-targeting polypeptide-nanobubbles (PP-NBs) with core size of 400 and 700 nm were fabricated and evaluated. With excellent physical property and specificity, PP-NBs of both core size could image PSMA expression in prostate cancer xenografts. Particularly, 400 nm PP-NBs generated higher PSMA-specific MRI/US dual modality contrast enhancement than 700 nm PP-NBs in correlation with histopathologic findings. Benefit from the smaller core size, 400 nm PP-NBs had higher permeability and specificity than 700 nm PP-NBs, hence producing better PSMA-specific MRI/US dual modality imaging.