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.