It's entirely random even if you set specific parameters - the only way you could really force those things to be very large would be to set a maximum on neutral tiles, and you can't do that. Just be patient and discard the maps that don't have what you want, you don't even have to gen them fully - just get to the point where history starts counting and hit Escape, and pan the map around, abort if it's junk. I've genned maps that have maybe half their tiles as good or evil, and with the same settings gotten more like 1%. You can fiddle with desired good/evil subregion tiles settings, but this isn't too helpful as this will cause worldgen to choke on excessive numbers of rejections for other reasons - I guess it will finish eventually, but when I've tried this it ran up to many thousands of rejections with a lot of reasons for failing. Since subregion alignment is determined at nearly the end of worldgen, just before civ placement and history, this means a lot of time is wasted on rejects because of all the shit done beforehand like calculating rivers and lakes. IMO quicker and cleaner to just roll many times and manually discard maps that you don't want.
Just for grins I'm actually trying to roll a map with very high min region sizes for good-aligned (1,000, 5,000 and 10,000 respectively), I'll come back when/if it ever finishes.
edit: that actually didn't take that long, although tbh the time it took was about the same as rolling it manually - however very nearly all the map is good aligned, which is pretty cool. The cruddy part is you have to babysit it to intervene on all the excessive rejections dialogs.
edit again: after a couple of times going through this, it's actually not too bad time wise, give it a try I suppose ^^ err, that's been with medium map size