The options before a growing church
The options open to a growing church are (a) grow bigger, (b) divide in two. Option (a) is nine out of ten times the route taken. There are some advantages of scale, no doubt, but the disadvantages, as I see it far far outweigh the advantages. I am speaking from a English context:
The disadvantages of a big church
An over-dependence on the preacher. Normally larger churches have good preachers (one reason they grow). But before long, the church becomes dependent on one man - not a good idea, and a terrible one when it comes to succession.
Passengers not particpants. As the church grows, the working or serving core remains static. New people think like this "No-one needs my gifts" or worse "Great I can hide".
The advantages of splitting
No dependence on big wigs (my own view of big wigs is that they should probably be functioning more like apostles across churches rather than as pastors within a single church).
All members have to use their gifts - I mean they just have to! You simply can't sit on the sidelines, because you are needed, like desperately!
Connected to the last, new gifts emerge. Gifts which would have remained in the shadows of a large congregation.
Apart from in Acts, we know next to nothing about the sizes of churches in the NT. No church is commended for being large, none condemned for being small. All we hear is exhortations to godly living and eager waiting for the final day.