The problem with feudalism is that there is a whole lot of confusion attached to how it was supposed to work and how it actually ended up working.

This is a generic description of how it supposedly worked, adapted from an earlier thread of mine:

Let's take King of France in the year 1000 (an arbitrary date selected by me) who cannot rule all of the kingdom on his own, so he divides it among his friends. This largest (top-level, what have you) feudal subdivision is a Duchy. In France, the traditional duchies were Burgundy, Normandy, and Aquitaine (plus the Archbishop of Reims who for all intents and purposes also ruled a duchy). In England prior to the Norman Conquest, this rank was called "Earl", while in Germany, he would be called "Herzog". Further, the ruler of Brittany once annexed by France was "demoted" to the title of Duke. There could also Marquis, which are technically the same rank as Dukes but rule border territories. Thus they have more responsibilities, but also more rights (this title is more present in Germany as "Magrave" and Italy, as "Marchese").

A Duke in the year 1000 in turn cannot conceivably rule his entire Duchy on his own, so he splits up the territory among his friends. This second subdivision is called a County, ruled by a Count or Viscount. The Duke of Normandy, for example, split up his Duchy in the counties of Rouen, Meulan, Evereux, Eu, Contentin, Cau, Bayeux, Avranche, Aumale, and Alençon. The Counts, in turn, would split up their counties among Barons. Barons granted land to Knights, who were given enough land and serfs to maintain arms, armor and a stable (not that Barons and Counts wouldn't fight as knights in time of war; again, the terminology gets messy fast).

Some counties (like the County of Toulouse) didn't belong to a Duchy, and were directly under the rule of the King (and in the case of Toulouse, the King later ruled it directly as his own possession from the thirteenth century on). Further, several Counts under the direct rule of the King who had either by purchasing or marrying (or even war) acquired large, near-Duchy sized territories were in time elevated by the King to the rank of Duke (Bourbon, Orleans, Anjou, Berry, Angoulême, Alençon, Touraine).

Keep in mind, a Count isn't supposed to be more powerful than a Duke, but it can and did happen.

Viscounts are weird creatures. The etymology is slightly different than that of Count, though the words are similar. Think of a Viscount as a count who's particularly chummy with the King/Duke.

And finally, if we're talking of an early modern king creating a title ex-novo he can call it whatever he wants, regardless of the size of the estate. However, many titles had long histories dating back to ancient Merovingian subdivisions among co-kings. Whomever was holding Aquitaine, for example, would want to be called Duke of Aquitaine.