Use inheritance laws to keep dividing up property
This is actually discussed by Adam Smith in his famous treatise on the Wealth of Nations. I don't remember what his ultimate verdict was, but he was comparing the inheritance laws of Europe with those of America. In Europe, typically the first-born son inherited the family estate. This had the effect that family estates lasted for a very long time, and tended to grow bigger rather than smaller. It also shuts out new buyers; even if you had the money, there simply wasn't that much land up for sale.
In America and maybe some other locations, inheritances were split equally between all heirs. This creates the effect over time that land holdings are broken up and re-aggregated over time as smaller pieces may be sold by heirs, or bought by neighbors. No one family could simply sit on its land forever. Fortunately, the West offered plenty of new land for settlement.
Three policies to accomplish your goal
For your purposes, you could build a world with inheritance laws that achieve the effect you want. First, a heavy inheritance tax (death tax) could make it difficult for heirs to actually keep their parents' property -- more difficult the larger the property. Your government might even claim its share in the form of land rather than cash, and auction off the land so the heirs would have to compete with other bidders for it. This both reduces the size of family estates and ensures a regular supply of land for sale to new buyers.
Second, impose an inheritance law such that, regardless of the wishes of the deceased, each heir receives an equal share of what's left after taxes. The deceased's last will would be seen as more of a request or suggestion, rather than being legally binding on the heirs. This would be more effective if you can establish that, culturally, families in this kingdom are rather large. When the resulting shares are too small to live on, some heirs will sell their shares to other heirs or to neighbors and move elsewhere or find a way to earn a living without land. Property holdings should grow and shrink fairly dynamically across the generations. My neighbor might buy some of my late father's land from one of my siblings, and I might eventually buy it back from one of his children.
Third, you might establish some incentives to use the land rather than keeping it idle. On the "carrot" side, you might create a Homestead Act that offers free land to citizens who will live on it and establish a farm or business. On the "stick" side, you might levy a property tax that applies only to land that is not put to productive use, encouraging large landowners to sell off their excess land. The effects of these incentives should be to encourage lots of people to own small homesteads rather than a few people holding on to large estates.