An experimental study is conducted to analyse the influence of tone hole (TH) size on the pitch of woodwind instruments. Time-resolved Particle Image Velocimetry (PIV) is used to measure the velocity fluctuations and the natural frequencies of the resonator inside a simplified recorder. The sound is generated using a real mouthpiece, and the airflow has the same intraoral conditions as during human performance. PIV measurements are validated with pressure measurements obtained from pressure transducers. Four tone hole diameters are considered, ranging from 0.3 to 0.8 times the diameter of the bore. The results show local peaks of the velocity fluctuations in the proximity of the TH. At the open end of the instrument, the amplitude of the velocity fluctuations reduces with the size of the TH. Finally, a quadratic law between the first harmonic of the natural frequencies and the TH diameter was observed.