Really you can configure them however you want as long as sims have at least a half tile access to one full side.
They will hold 4 small regular plants, or 1 tree, and one planting of an oversized crop, and by that I mean when you sprinkle oversized crop seeds onto a patch it initially divides into 9 mounds. Depending on how you care for those the will consolidate into their crop size:
- 4 mounds = small
- 2 mounds = medium
- 1 mound = giant
This happens only once after you have one of the above number of plants they will not divide down further. Gardening skill, fertilization, and the amount of attention you give to the 9 mound initial planting stage will help increase your odds of achieving a giant crop. You will know your oversized crop is done growing when you see the option to harvest on the interactions wheel. As far as I can tell they never go bad, and are always in season. Oversized crops won’t grow in planter boxes.