So looking at that, a 2 tile wide LandBridge would take a fraction more than a one tile wide one upto an average depth of 70 dirt. So if the deepest part is over 100 deep, then i would recommend a 2 tile wide one.
The decision for the View Vires LandBridge was for a 2 tile wide one as it looks a lot better than a single tile (also some tiles were a lot deeper than 70). It's also easier to slip off a one tile bridge, so a 2 tile bridge is more convienient to use.
Enough of deciding what type of LandBridge to go for... Here is how to do one