What is the length and width of a 864 square feet

The area doesn't tell you the dimensions. There are an infinite number of different answers, all with the same area but different dimensions.

Here are a few:

1 ft by 864 ft

2 ft by 432 ft

3 ft by 288 ft

4 ft by 216 ft

8 ft by 108 ft

16 ft by 54 ft

24 ft by 36 ft

27 ft by 32 ft.

The area of each of these is 864 square feet.