100 stands for "100%" and it is the speed of the ufo. Check the rusleset reference: https://www.ufopaedia.org/index.php?title=Ruleset_Reference_%28OpenXcom%29
The first coordinate (1, 1, 100) is the "enter point", it is needed but, as you said, it immediately lands following the instructions given by the second (1, 0 , 100) coordinate.
Yup, somehow I got confused with that 100 setting.
About the size of the maps: frankly I think that UFO map sizes are perfect. Bigger maps means the missions will drag on too long, especially when hunting down the last alien alive hidden somewhere and you have only few men left (because of the casualties you had during the mission) to search for it. TFTD already had maps too big (Gollop said so too and I agree). So I personally would be VERY careful about adding "big maps".
This is a very interesting discussion.
The issue with TFTD's maps that suffered from that problem (ship attacks, artifact sites and alien colonies) had less to do with their sizes, but more with being multi level missions and how the maps and AI were designed (too cramped, making it harder to find the last alien). Considering that Gollop enlarged slightly the map sizes on Apocalypse (specially using more levels) but at the same time redesigned the AI to make it more likely to meet the player says a bit of how these different factors play out.
Another main reason on why I prefer to use slightly enlarged maps (60x60x4 on the terrain pack) is my experience with Enemy Unknown 2012 and UFO2000. On EU2012 the maps are, on average, bigger than those used in UFO but at the same time having less units. The combat system is different sure, allowing for more movement and preventing most uses of free firing. But at the same time, with UFO2000 (the multiplayer version of UFO, with the same combat system) players eventually settled on the best battle conditions being 6-8 units per side on a 50x50 map, so the combat system isn't the most important factor.
Cramped maps (lots of small rooms, hard to reach locations, built in levels, etc.) are also a no-go. I learned that by designing several terrains that way (Hive, The City) when I started modding and I will not include those on the Terrain Pack for that reason. The interesting thing about TFTD is while it had several maps designed this way, it also included some terrains exactly the opposite, like Port (which I consider the best terrain of both UFO/TFTD) or Island, but those tend to be overlooked by the chore of having to clear all the rooms of the Cruise Liners.
The conclusion I draw from all of this is really that slightly enlarged maps (60x60) can be better than the current map sizes due to the squad sizes in UFO, by allowing more movement and less '1 large group advancing and shooting non-stop to targets at the distance from the moment they land' (which is the most effective tactic in most maps but that gets boring after a while) if properly designed.
