Mark's garden area is 32 square feet.
Steve's Garden at 128 square feet will be 4 times as large and similar in shape.
For similar polygons if ratio of sides is a:b then ratio of areas is a^2:b^2 or conversely, if ratio of areas is a^2:b^2, the ratio of the sides is a:b.
Since the ratio of the areas is 1:4, the ratio of the sides will be 1:2, so Steve's garden will be 16 feet long by 8 feet wide.