This sounds like more of a class distinction than a land size issue. 
Plantations belonged to the Gentry and the farms belong to the peasants. 

John Philip Adams

-----Original Message-----
From: Discussion of research and writing about Virginia history
[mailto:[log in to unmask]] On Behalf Of Joan Horsley
Sent: Thursday, September 27, 2007 6:58 PM
To: [log in to unmask]
Subject: Re: land definitions

I'm interested in your distinction between "plantation" and "farm," because
I don't think I've ever seen the word "farm" in pre-Revolution northern
Virginia deeds, road orders, or wills, only "plantation." I had assumed that
"plantation" in such records was a generic term for cultivated land
regardless of crop, size, etc--at least that's how it seemed to be used.
  
There's this example from a 1710 document regarding the distribution of land
to the Huguenot refugees of Manakin. Here again "plantation" seems just to
mean "farm"  or even simply a piece of land that could be farmed:
  
"And it is further Ordered, that such of the French Refugees as have bought
the Plantations, or dividents of any other of the said Nation in the first
5,000 Acres, shall have and enjoy the same without prejudice to such
Purchaser, to hold the Land due to him for his own share, and to take up as
much more as will make his said Share the Compleat quantity of 133 Acres.
[R. A. Brock's Huguenot Documents] 
  
Could it be that, at least early on, the use of "plantation" differed
depending on the context--a kind of value-neutral "place of planting" for a
land or court record versus a social situation with cultural and class
implications of heirarchy? Or have I misunderstood the records?
   
  Joan Horsley
  
Harold Gill <[log in to unmask]> wrote:
  Plantations in Virginia produced a staple crop such as tobacco while farms

produced provisions such as corn, wheat, and other grains.William Nelson 
wrote that his son Hugh converted one of his plantations to a farm for 
growing provisions instead of tobacco. John Adams wrote in Feb. 1777: "The 
Planters are those who raise Tobacco and the Farmers such as raise Wheat 
&c." This distinction seems to have been normal in 18th century Virginia.
HBG