In a way, defining the size of a knight's fee is nonsense. A bit like answering the question, 'How big is your mortgage?' with, 'Some land and a house big enough for my family.'
The better question to a lord in medieval times would be, 'How much land do you hold to support your knight's fee?' And if you did manage to ask a dozen manor lords in the 12th or 13th century, you would probably get a dozen different answers. Adding to the confusion, each of the dozen might say he held a different number of virgates (or yardlands). At that point, one might be totally confused because a virgate had no standard size.
However, at least originally, a general rule was that a knight's fee required as much land as it took to support one knight, his family, and sufficient armaments enabling the lord to serve in the king's army. This applied whether or not the individual holding the land with attendant knight's fee was a knight in the military sense. A much greater total land area would be necessary to support a knight's fee in a location of poor quality soil, perhaps interspersed with marshlands or ponds. Conversely, a smaller total area was required with only high-quality arable land.
So all this tends to turn the question around, because the 'mortgage', or obligation, was relatively constant. What varied was the total land area necessary to fund that constant obligation. However, the story did not end there! The king (or a baron) was not by any means so even-minded as to consider only the cost of supporting a fighting soldier. Sometimes he simply felt generous towards someone for whatever reason, or conceivably, malicious enough to award less land than needed for the obligation of a fee or part fee.
A long time ago, a high-ranking official of the king's court provided one basis for this conundrum; he did so because the knight's fee became the basis for taxing landholders on a national basis. Instead of a fee, or scutage, per knight's fee, the tax became a fee on each carucate of land held. One knight's fee, he said, was four carucates (or hides), each carucate was four virgates, and, he went on, each virgate was 25 acres. Which made 400 acres. A few centuries later, another measure had a virgate at 40 acres, or 640 to a knight's fee. Others thought 800 acres was correct.
But then again, each of the 640 (or other measure) acres was arable, because a virgate was strictly arable land. There was no restriction on the number of acres of meadow or pasture that inevitably went with the arable land. Watford parish, an example of one knight's fee, in total encompassed 3,750 acres; Watford town alone was over 2,000 acres. And over time the parish was evaluated for tax purposes as anything from two hides to 25 hides.