Motocross or scramble tracks on private property for your own use can start at roughly 1/2 acre. But you are best to acquire an area of land of an acre+ to create a reasonable sized personal motocross track. One acre of land will allow you to have features that include undulating land with dips, mounds and jumps. Planning permission may be required for the creation of the man-made jumps. Think about noise levels and the location of your motocross or scramble course.