We are not told how big the crowd was. Estimates regarding size depends largely on where Peter was when he spoke. In that regard we can only confirm that he was in an upper room together with the apostles (see Acts 1:13). The fact that Peter stood up to address the crowd suggests either that the people came into this upper room or that he was seated outside with the apostles in a more public place. One final detail that has some bearing on this question is the fact that about 3,000 people were baptized and joined the church before the end of the day (Acts 2:31). This suggests a relatively large crowd was present or there must have been several speeches throughout the day to lots of smaller gatherings.
14 But Peter, standing with the eleven, lifted up his voice and addressed them: “Men of Judea and all who dwell in Jerusalem, let this be known to you, and give ear to my words.