Default buffer size is 4 KiB.
Also, take a look here: Sequential File Programming Patterns and Performance with .NET
Programming patterns for sequential file access in the .NET Framework
are described and the performance is measured. The default behavior
provides excellent performance on a single disk – 50 MBps both reading
and writing. Using large request sizes and doing file pre-allocation
when possible have quantifiable benefits. When one considers disk
arrays, .NET unbuffered IO delivers 800 MBps on a 16-disk array, but
buffered IO delivers about 12% of that performance. Consequently,
high-performance file and database utilities are still forced to use
unbuffered IO for maximum sequential performance. The report is
accompanied by downloadable source code that demonstrates the concepts
and code that was used to obtain these measurements.