In Farming Simulator 19 you can create new fields, merge existing fields and "cut" existing fields. You can only create new fields on a previously purchased plot. Activate in the plough (it cannot be a subsoiler) the option to modify the area with Y key (XONE: LB+Y; PS4: R1+triangle), and you will adjust the lesser fields to your requirements and increase their area for free.
To reduce the area or divide the field into smaller areas, use the Meadow Roller Varioroller (the Miscellaneous tab in the Store). The roller must be unfolded before use and hooked sideways. The ploughing up of a piece of field and the use of a roller creates a "neutral" surface - a meadow. This will also remove the shrubs.