As far as I know...

There was a general agreement that the background for the big map should be black so the map wouldn't look like granny's patchwork quilt with all our different background patterns merged together.

I believe the ideal size for maps presented to Red for inclusion is 100 px per 5' grid square, which is the resolution of the grid that John provided (I think he did it for me because I was pretty confused for a while, but he was too sweet to point the finger at me ) the grid could be used as a convenient place to start, but you don't have to use it. That's not a rule. Personally, I'm working at 150 px per 5' grid square and reducing the final image to 100 px.

The reason we are working at a maximum image size of 2000 px completely escapes me, but I do remember sweating a lot over a lot of numbers that Red gave us, and that 2000 px square seemed to be the ideal maximum size. It makes sticking this thing together into the big zoomy map a bit easier for Red in the long run - and the amount of work he's doing I think its definitely worth trying to stay with that number, but again - there's no hard fast rule about that, and your map won't be disqualified or anything if its bigger - especially not if its drop dead gorgeous

Corridors/paths/tunnels leaving the map would be best done at right angles to the edge they are crossing, and be either 5' or 10' wide (1 or 2 grid squares). That's so they match up with other maps a bit better.

Its also best if you leave a bit of space around the outside of your map so that the exits stick out just a bit. That way, Red can use the bits that stick out to merge exits from different maps together more smoothly.

Where the map is very close to the edge - just because it happened that way, or whatever - you may need to make a separate bit of exit passageway to join it to another map.

I think that mostly sums it up