The first church, documented in Acts, had 3,000 male members. If you include the wives and children it may have been as large as 10-12,000 people.
So there is precedent for the Mega church.
Only if "The Church" met at one location. I think you'll find that they didn't, and it still doesn't.
They devoted themselves to the apostles teaching and to fellowship, to the breaking of bread and to prayer.
Everyone was filled with awe at the many wonders and signs performed by the apostles.
All the believers were together and had everything in common.
They sold property and possessions to give to anyone who had need.
Every day they continued to meet together in the temple courts. They broke bread in their homes and ate together with glad and sincere hearts, praising God and enjoying the favor of all the people. And the Lord added to their number daily those who were being saved.
Their life was about so much more than a big "happening" on Sunday morning.
So there is precedent for the Mega church.
I wouldn't say that there's precedent for mega churches. God sent Saul to that mega church in Jerusalem.....
We are to go out into all the world and preach the gospel. A church that's too big tends to be ingrown.