[parhamn]

According to wolframalpha (Log2(surface area of earth / 9m^2)) about 46 bits of information.

[vincnetas]

So basically using A-Z 0-9 you could have it geocoded using 3 x3 signs (for example 2J4.KI9.999)

[ignoramous]

One can be more cute and come up with https://en.wikipedia.org/wiki/Open_Location_Code