Thanks! Nightghaunt :)
On hexes size: I recommend choosing between 6miles to no larger than 12miles, according to the crawling rules you're using and the scale of the map you generate. I tend to go for 6 miles hexes when generating a large map (>80 hexes) or 12 miles when generating a smaller map.
Good point on providing more explanations for the generator parameters:
The Biomes placement experimental selector allows controlling how close similar biomes regions are packed together on the map. Packing regions together works better when you have relatively smaller hex count per region and smaller scaled maps.
The probability multiplier baseline chance is 1 in N options in each category.
About re-rolling - this is a tricky one and requires a brief overview on how Hexroll works internally - but the short version is that implementing this will require re-working the core of the generator.
One way of explaining how Hexroll works is by using a painting analogy. Just like a painter layers a group of brush strokes to create an effect or texture, Hexroll layers elements of the sandbox one on the other to link and tie things together. Let's take re-rolling a dungeon room as an example. Let's say the room in question has a missing NPC in it. If this room is to be re-rolled, we might lose this piece of information about it, and either have a missing piece in the sandbox, or we'll be forced to regenerate other parts of the dungeon - causing an even bigger chain reaction.
Now, I specifically chose the more difficult case, and there are certainly things that could be re-rolled without affecting much. I should spend more time thinking about this...