Get file's size from bytes array (without saving to disc)

What about array.Length? Looks like a size in bytes.

Um, yes:

int length = byteArray.Length;

A byte in memory would be a byte on disk... at least in higher level file system terms. You also need to potentially consider how many individual blocks/clusters would be used (and overhead for a directory entry), and any compression that the operating system may supply, but it's not clear from the question whether that's what you're after.

If you really do want to know the "size on disk" as opposed to the file size (in the same way that Windows can show the two numbers) I suspect you'd genuinely have to write it to disk - and then use a Win32 API to find out the actual size on disk.

Array.Length would give your total size expressed in byte.
Physical dimension on disk may be a little bit more considering cluster size.