Sift together flour, baking soda, and baking powder in a large bowl; set aside. Put butter and brown sugar into an electric mixer fitted with the paddle attachment; mix on medium speed until fluffy. Mix in spices and salt, then egg and molasses. Reduce speed to low. Add flour mixture; mix until just combined. Divide dough in half, and shape each half into a disk. Wrap in plastic wrap, and refrigerate until firm, at least 1 hour.
Line a 16-by-14-inch baking sheet with parchment paper. Using a plate or cake pan as a template, draw an 8- to 9-inch circle on the parchment; set aside.
Remove 1 piece of dough from refrigerator; let stand at room temperature until slightly softened, 5 to 10 minutes. Roll out dough on a piece of lightly floured parchment to a 1/4-inch thickness. To prevent dough from sticking as you roll it, loosen the dough from the work surface with an offset spatula and dust with flour as needed. Freeze until very firm, about 15 minutes. Repeat with remaining dough.
Preheat oven to 350 degrees. Remove dough from freezer. Working quickly, cut into gingerbread-man shapes with a 3-inch cutter. You will need about 18 in all. If your dough becomes too soft, freeze for 5 to 10 minutes. Transfer cookies to baking sheet. Freeze until firm, about 15 minutes
Arrange gingerbread men around circle, centering on the line and overlapping slightly. Lightly brush between overlapping hands and feet with a wet pastry brush; gently press together. Sprinkle with sanding sugar. Freeze until firm, about 15 minutes.
Bake, rotating halfway through, until crisp and golden around edges, 13 to 15 minutes. Let cool completely on sheet on a wire rack. Weave a ribbon through wreath; hang away from heat or moisture. The wreath will last up to 2 months.
3 cups all-purpose flour, plus more for work surface