For laughs, I tried to see what a very cramped world would look like, with otherwise default settings. I set Nauvis hex size to 8 and stroke width to 2 (the minimum allowed values), and received a fatal error:

Error while running event hextorio::on_chunk_generated (ID 16)
hextorio/api/axial.lua:642: table index is NaN
stack traceback:
hextorio/api/axial.lua:642: in function <hextorio/api/axial.lua:593>
(...tail calls...)
hextorio/api/dungeons.lua:345: in function 'spawn_entities'
hextorio/api/dungeons.lua:296: in function 'spawn_hex'
hextorio/api/hex_grid.lua:814: in function 'init_dungeon_hex'
hextorio/api/hex_grid.lua:1003: in function 'initialize_hex'
hextorio/api/hex_grid.lua:2235: in function 'initialize_adjacent_hexes'
hextorio/api/hex_grid.lua:2199: in function 'on_chunk_generated'
hextorio/control.lua:254: in function <hextorio/control.lua:229>

Seems that dungeon generation fails?

I suppose stroke of 2 is to prevent cases of accidental overlap and allowing the player to physically run unclaimed hexes using literal edge cases.
However, if each hex has its own stroke of 1, then diagonal skipping should still be impossible - I think.

If the issue comes from dungeon generation, perhaps the 2-3 wall thickness can also be adjusted so it's not as wide on extremely small hexes?
Settings seem to work with size=12, stroke=2

I fixed the crash for 1.7.4, although gameplay seems to be quite difficult on that setting. It'd be a real challenge run.