In Dragon Quest Builders you can control how big your fields to grow crops are. However, the quest in the early part of the game which you are asked to do this requires that you’ve met a special character. Wrigley the worm has the ability to create fields of soil that is suitable to grow crops.
Wrigley will be another of the quest markers that are eventually added to your map, so if you don’t have his location you basically have to perform other tasks until it shows up. Once you’ve added Wrigley the Worm to your party and brought them back to Furrowfield, increasing the size of your fields is a matter of laying down scarecrows and placing them in spots where you want Wrigley to clear.
The Scarecrow can be made at the crafting table and then can be placed anywhere there is available space. You will need some specific ingredients to craft the Scarecrow. This requires 1 block of wood, 1 unit of cord, and 5 units of dry grass. Wood can be found by cutting down trees, dry grass can be found from hitting grassy patches, and rope can quickly be found by smashing vines.