Well, it depends on the source (video) and if it was already compressed before. There are files that I can crop a lot at the same level and there are other videos that I can crop very little. I use Handbrake adjusting and tuning to -23-24-25. Testing at 23,24, 25, and I keep the one with the least loss at a reasonable size.
But there are other tools and I am not a professional either.