Different perspectives, different answers
If you are after fame, the answer is at the infinity end of the spectrum: the bigger, the more well known you will be; golly you'll soon be on the Telly. And in our fame-mad culture, glory is all that really matters.
But what might be a New Testament approach to the question, When is a church too big?
When real discipleship is no longer taking place
The commandment of Christ is to make disciples of all nations teaching them to obey everything he commanded them to (Matthew 28). The process Jesus used, if we note carefully his method, was to gather believers into small groups - i.e. The Twelve - who would teach one another and learn from one another as well as learn from Him. Over time, they would, by the Spirit and Word, integrated into the body of Christ, become more like their Master.
When believers are no longer maturing in Christ - advancing in knowledge and Christlikeness - then the church is too big.