I have searched some posts and cannot find what the maximum filesize is under iPhone.
According to the following links I found,
As the first link says, "The theoretical maximum file size for a Mac OS Extended file system is millions of terabytes. In practice, the maximum file size is equivalent to the maximum volume size, except for a small amount of disk space reserved for file system information."
Because the maximum filesize is equal to the maximum volumn size, and consider the factor about the free disk space.
So, in my conclusion, the maximum size of single file depends on the free disk space.