Mixed consists prevail. The sky is the limit, so go long. You can run the greater sized steam and diesel locomotive, plus full-scale passenger cars. Long trains really do look great on a large pike. That doesn't mean the little guys are gone forever.
Smaller trains do well in freight yards and smaller railways attached to the main pike.
Some folks buy whatever strikes their fancy, and that's what they run. Some are strict adherents to matters of scale and prototype consistency. There are those who invent their own trains, which range from amusing to outright wacky. Most of us are well within these extremes. No matter how wacky or mundane your railway, it helps to have a sense of order keeping it all together.