In a transitional boundary layer subjected to free-stream turbulence, streaky structures develop and eventually break down to turbulence after wavy motions. The purpose of the present study is to reveal the dominant scales of the streaky structures using a novel stereo PTV system measuring the streamwise and wall-normal velocities. The correlation of the streamwise velocity exhibit a negative peak at a certain spanwise separation. If normalized with the local boundary layer thickness, the spanwise separation minimizing the correlation decreases downstream and reaches the boundary layer thickness at the most downstream position. The streamwise correlation coefficient is asymptotically decreasing to zero and the streamwise integral scales are increasing downstream, indicating the large streamwise elongation of the streaky structure. The normalized spanwise scale of the wall-normal disturbance velocity also decreases downstream and it is always narrower than the width of the disturbances of the streamwise velocity. In contrast to the elongating streamwise velocity disturbance, the integral scale in the streamwise direction of the wall-normal disturbance is constant with downstream position. The length scales of the wall-normal disturbance are more less constant with the streamwise position and this result concludes that the wall-normal disturbance is simply convected by the mean flow after the receptivity process.