How do you decide what byte[] size to use for InputStream.read()?

Most people use powers of 2 for the size. If the buffer is at least 512 bytes, it doesn't make much difference ( < 20% )

For network the optimal size can be 2 KB to 8 KB (The underlying packet size is typically up to ~1.5 KB) For disk access, the fastest size can be 8K to 64 KB. If you use 8K or 16K you won't have a problem.

Note for network downloads, you are likely to find you usually don't use the whole buffer. Wasting a few KB doesn't matter much for 99% of use cases.

In that situation, I always use a reasonable power of 2, somewhere in the range of 2K to 16K. In general, different InputStreams will have different optimal values, but there is no easy way to determine the value.

In order to determine the optimal value, you'd need to understand more about the exact type of InputStream you are dealing with, as well as things like the specifications of the hardware that are servicing the InputStream.

Worrying about this is probably a case of premature optimization.