Counties
Counties are sections of land between 1 km^2 and 256 km^2 in area. An average county consists of 2,300 parcels, or around 3 km x 3km. During Domain Selection, two or more counties can be combined into one by Counts with more than one nobility package.
Smallest County
1 km x 1 km (258 Parcels)
(Image Credit: Google Maps)
A county this size would only be about three times bigger than the smallest Capital (81 parcels)
Large County
4 km x 4 km (4,128 Parcels)
The average county was listed as 2,300 Parcels, which is 3 km x 3 km. It is looking like that is indeed the average parcels-per-county in most domains.
(Image Credit: Google Maps)
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Duchies
An average Duchy filled with average Counties (2300 Parcels) will have an area of around 226.1 km^2
An equal kingdom (see below) will be made up of 12 equal duchies, each being 256 km^2
24 equal counties within an equal duchy would each be 10.667 km^2 (2,752 Parcels)
For comparison, the Life is Feudal MMO map is about 450 km^2 (116,101 Parcels) [including waterways]
The 16 km x 16 km (256 km^2) picture in the below section represents an equal duchy.
Single Duchy
256 Square Kilometers (66,049 Parcels)
The largest duchies are around this size, with the majority of duchies being smaller.
*Note that the upper left corner of this county is in the same location as the upper left corner of all the following images (Image Credit: Google Maps)
Double Duchy
512 Square Kilometers (132,098 Parcels)
(Image Credit: Google Maps)
Triple Duchy (3 equal duchies)
768 Square Kilometers (198,147 Parcels)
(Image Credit: Google Maps)
Large Triple Duchy
1024 Square Kilometers (264,196 Parcels)
A Duke can only select 3 duchies at Domain and Settlement Selection. Finding 3 extra large duchies that border each other will be rare (Image Credit: Google Maps)
In CoE, settlements will likely be closer together than seen in these images.
Things to look for:
Mississippi River running vertically on the left side
Wisconsin River running diagonally across and emptying into the Mississippi
Where farms are, where farms aren't
Where major roads are built and why
Dispersion of towns
Transition between hilly and flat terrain
For future reference, note the intersection of Highway 18 and Highway 80 near Montfort and its relation to the bottom right corner of the duchy.