Free Sample, how soon we forget!
I refer to the days you walked into a book store and picked up a book, read the blurbs, maybe read part of a chapter before making the "buy" decision.
Now, the Author wants top dollar for a book, sight unseen?
With Internet and E-Books, the "Sample" replaces the thumbing through process.
No sample... (probably) No Sale
Some of my favourite authors style changed over time. Sometimes it diverged from my tastes. Sometime, my tastes changed
How big a sample? Big enough to get a sense of Style, and Story (first impressions count). Usually a chapter or 2 should do it as a sample. Depends The author should suggest how much to sample.
Baen has the right idea. Samples work for print books also.