My apologies if this isn't the best place to ask, but what are the pros & cons of going big or small on world size and history length?
In my experience, the pros are speed. with a smaller world and fewer historical figures you typically get faster saves, faster loads, higher FPS.
The cons are mostly opinion, and vary depending on your goals. If you want historical figures, not having them is a con.
If you want to travel farther/longer, or have particular features in your world, sometimes a larger world is necessary.
However, for a certainty, a larger world with a huge number of historical figures is more like to have slower loads, slower saves, and lower FPS, all other things being equal.
In general, I find that if players are primarily interested in playing fortress mode, a massive world with 99% of it outside the range of the fort has no value, but YMMV.
You can, however, have massive worlds with no historical figures, and have reasonable FPS, and not super annoying load & save times.
Similarly, you can have a pocket world with insanely high numbers of historical figures and have ridiculously long save & load times, with abysmal FPS.
Once in fortress mode, pathfinding can crush an otherwise fine fort, as time passes, and various bugs and behaviors begin to appear. Even things like the number of dead, or the number of items in a fort, or water flow, or magma flow.. taken to absurd extremes, each of these can crush a world down to single digit FPS.
Happily, DF allows you to experience the entire range of good to bad, within your own personal tolerance of good and bad.