What is the biggest shadow that a cube of side length $1$ can have, with the sun at its peak?
Note: "The biggest shadow of a figure with the sun at its peak" is understood to be the biggest possible area of the orthogonal projection of the figure on a plane.
1. **Understanding the Problem:**
   We need to find the maximum possible area of the orthogonal projection (shadow) of a cube with side length 1 when the sun is at its peak. This means we are looking for the largest possible area of the shadow of the cube on a plane.

2. **Simplifying the Problem:**
   Consider a cube with side length 1. The cube has 6 faces, each with an area of 1. When the sun is directly overhead, the shadow of the cube on the ground is the orthogonal projection of the cube onto the plane.

3. **Projection of the Cube:**
   The orthogonal projection of a cube onto a plane can vary depending on the orientation of the cube. The maximum shadow area occurs when the projection is maximized.

4. **Choosing a Vertex and Adjacent Faces:**
   Let's pick a vertex \( P \) of the cube and consider the three faces that meet at \( P \). These faces are adjacent and form a corner of the cube. The projection of these three faces will give us an insight into the maximum shadow area.

5. **Analyzing the Projection:**
   The three faces adjacent to vertex \( P \) can be considered as forming a tetrahedron \( PABC \), where \( A, B, \) and \( C \) are the vertices adjacent to \( P \). The shadow of the remaining three faces is double the surface area of the tetrahedron \( PABC \).

6. **Calculating the Area of the Triangle \( \triangle ABC \):**
   The area of \( \triangle ABC \) can be calculated as follows:
   \[
   \text{Area of } \triangle ABC = \frac{1}{2} \times \text{base} \times \text{height}
   \]
   Since \( A, B, \) and \( C \) are vertices of the cube, the length of each side of \( \triangle ABC \) is \( \sqrt{2} \) (diagonal of a face of the cube).

7. **Using the Formula for the Area of an Equilateral Triangle:**
   The area of an equilateral triangle with side length \( s \) is given by:
   \[
   \text{Area} = \frac{s^2 \sqrt{3}}{4}
   \]
   For \( \triangle ABC \), \( s = \sqrt{2} \):
   \[
   \text{Area of } \triangle ABC = \frac{(\sqrt{2})^2 \sqrt{3}}{4} = \frac{2 \sqrt{3}}{4} = \frac{\sqrt{3}}{2}
   \]

8. **Maximum Shadow Area:**
   The maximum shadow area of the cube is double the area of \( \triangle ABC \):
   \[
   \text{Maximum Shadow Area} = 2 \times \frac{\sqrt{3}}{2} = \sqrt{3}
   \]

The final answer is \( \boxed{ \sqrt{3} } \).