How big is 0.18acres in square feet or for a building lot how wide and long is it?

1 acre = 43560 sq ft
→ 0.18 acre = 0.18 x 43560 sq ft
= 7840.8 sq ft.

There is no definite answer for the length and width of a building lot as the building lot can have any length and width as long as

its area

(length x width) is 7840.8 sq ft.

If it is a square building lot, it would be approx 88.55 ft by 88.55 ft.
However, if it is rectangular, it could be approx 98 ft by 80 ft, or 131 ft by 60 ft, or 196 ft by 40 ft, or 157 ft by 50 ft, or even 7480 ft by 1 ft!