This is a hard question to answer because the Dyckman family started acquiring land in Northern Manhattan in the 1660s and continued to increase their holdings until the mid-19th-century. The Dyckmans were continually buying and selling property so, the boundaries changed depending on the year.
The mythology that we commonly hear is that the Dyckman family owned all the land north of 125th Street. This is simply not true. What is true is that there were lots of Dyckman descendants who owned property in Northern Manhattan. For instance Jacob Dyckman, brother to William who built the farmhouse, owned property at what is now the corner of Central Park and East 105th Street. In the 17th century, a Dyckman in-law owned the property on which Morris Jumel Mansion now stands.
For our purposes, we consider the land that was held by William and his descendants as the official Dyckman farm with the c. 1784 farmhouse at the center. In a 1787 ad the property was described as being 250 acres and in 1819 those boundaries were very roughly river to river on the east and west, from present-day 213th Street on the north and present-day 190th Street on the south.
The farm in 1819-1820 also included other houses, some lived in by family members and others by renters or workers on the farm. The land also included outbuildings such as barns, stables, a cider mill and fishermen’s houses along the Hudson River.
William’s son Jacobus added to the landholdings as did his grandsons Isaac and Michael. By the 1860s the family held over 340 acres of land in the neighborhood as well as land elsewhere in Manhattan and the Bronx. Isaac Dyckman, the last of Jacobus’ living son’s, died in 1868. In his will he dictated that the bulk of the family land in Northern Manhattan be sold and the proceeds go to his many nieces and nephews. The farmland was sold at four separate auctions between 1868 and 1871.