On the Swords & Wizardry forum here a question what asked on how size different realms on a map. This is my answer
Fundamental unit of feudal societies this is typically 1,500 to 3,000 acres supporting a landower household, 5 to 10 yeoman/crafter/freemen and 15 to 30 households of peasants. Household average about 5 individuals.
A barony is usually anchored by a Keep or Castle and is surrounded by 10 to 15 estates. 2/3 are run by vassal knights The Keep and Castle is either a large village or small town depending on it's location in relation to trade routes typically will have from 75 to 200 households. The Estate surrounding the Keep or Castle will be towards the higher end of the scale say from 2,200 to 3,000 acres. There will be a small number of urban households, more crafters, freemen, and yeomans.
A count is originated from a Roman military office of the late empire. A count was in charge of a protection of a region. In a feudal society a count will have between 1 to 4 Barons as vassals. The Count's personal residence is usually a castle and setup similar to the Baron's above.
Another late empire military office transformed into a noble titles. Dukes will have up to a half-dozen major vassals (Barons, Counts) along with 0 to 3 other keeps and castles that are directly ruled by the Duke and administered by a bailiff. The Duke's nearly always resides at a castle and it is usually a large town or small city.
A region that is a borderland between one realm and another. It varies in the number of baronies however a Count Palantine or a Duke Palantine is placed in charge. Sometime a Warden which is a royal officer is appointed to administer the territory in the name of the King.
A Duke that probably an independent realm.
The same as a Grand Duchy except with a fancier title.
A realm ruled by a King. The best way to estimate it's size is to figure how many baronies there are. After that you can arrange them in a feudal hierarchy. Remember that Kings can have Duke, Counts AND Barons as vassals. So a Baron may be the vassal of a Count or could be a vassal of the King. Make it as simple or complex as you wish.
The classic empire consists of a metropole or the heartland ruling a number of conquered peripheries (other realms). Imperial Administration consists of representatives of the emperors journeying or living in the periphery realm making sure the local render proper tribute and do what the emperor tells them to do. Colonies may be established throughout the periphery to give imperial troops allies and supplies in the case of revolt.
The barony and estate are the fundamental units you want to use to size your regions and realms. Generally a 2.5 mile hex will have one estate. So a Barony with 15 estates will cover 16 hexes. The 15 estates plus the Baron's castle estate. Generally a Barony will occupy a distinct geographical region like a valley bounded by hills. Also remember that feudal societies tend to disperse the estates of great nobles to prevent them from having a continuous territory.
So in the region of the Barony of Applegate there may be 15 estates. 5 of which are royal estate run by a royal knight or balliff. 10 are owned by the Baron of Applegate. 8 of which he granted to his knights and 2 are run by baliffs in his name. The baron has 5 other estates, 2 are in the Duchy of Newbourne, 2 in the Barony of Oppeln, and the 1 in the County of Westmarr. 3 of these estates are given to his knight while the other 2 are ruled by baliffs in his name.
100 acres = .156 sq miles.