What is the linear dimentions of 40 acres?

You can't tell the linear dimensions from the area. There are an infinite number of shapes that

all enclose 40 acres but have different linear dimensions.

The smallest possible straight dimensions that can enclose 40 acres occur if the field is square.

Each side would be 1,320 feet, and you'd need exactly 1 mile of fence to enclose it.

But if some developer owned a rectangular piece of land that was 330-ft wide and 1 mile long,

his land would also measure 40 acres, but it would take 2-1/4 miles of fence to enclose it.