Im not a map expert but i play a lot of singleplayer...
3. for size, i usually choose 6k, as its "enough for me" and i never see the whole map. Its also faster to generate and i have memory issues when generating larger maps anyway. But if you can go larger i believe the "cities and towns" will be a but nicer sized and more if them.
4. Since map generation times are so fast now (like a few minutes now and not 45 minutes like in the past), i always choose to make my own maps (to keep things "fresh"). However i usually dont care if there are hills or not, there are settings in the map generator that might help keep it "flatter" even if this is just the "map previewer"
5. i frequently choose these vanilla setting changes in all my games (and several others but these are the ones i almost always change)
- make blood moon 1 or 2 days "variance" so its not always the 7th day. I forget the setting name, but its right beneath the bloodmoon days setting.
- bump up all zombie day and night speeds 1 notch, so they never walk.
- usually i bump up player block damage 1 notch. So tree chopping/mining is less of a pain (in singleplayer).
- i set loot to never respawn.
- i set the game to private, as by default your game is visible and "open" to anyone who might find it in the game browser. I also set a password of gibberish just to make sure no one finds it and joins easily.
- i so far have found "feral sense" a bit too much in my single player games so i leave it off. But i also add many mods to make the game harder so without mods maybe its a decent challenge?