Given a convex figure in the Cartesian plane that is symmetric with respect of both axis, we construct a rectangle $A$ inside it with maximum area (over all posible rectangles). Then we enlarge it with center in the center of the rectangle and ratio lamda such that is covers the convex figure. Find the smallest lamda such that it works for all convex figures.
1. **Maximal Rectangle Centered at the Origin**:
   - Given a convex figure symmetric with respect to both axes, we need to find the maximal rectangle \( A \) inside it.
   - Suppose \( A \) is not centered at the origin \( O \). The coordinate axes divide \( A \) into four smaller rectangles.
   - If \( O \) is not the center, these four smaller rectangles are not all equal. Take the one with the largest area and consider a new rectangle \( R \) centered at \( O \) with this largest rectangle as one quarter of it.
   - \( R \) will fit inside the convex figure, but its area will be greater than \( A \), which contradicts the assumption that \( A \) is the maximal rectangle.
   - Therefore, \( O \) must be the center of \( A \).

2. **Symmetry and Convexity**:
   - Due to the symmetry of the figure with respect to the \( x \)-axis and \( y \)-axis, we can focus on the first quadrant.
   - The part of the figure in the first quadrant is also convex.
   - Let \( M = (a, b) \) be the point such that the rectangle with one vertex at \( O \) and the opposite vertex at \( (a, b) \) has maximal area \( k \).

3. **Tangent to the Hyperbola**:
   - Consider the hyperbola \( y = \frac{k}{x} \).
   - Let \( l \) be the tangent to this hyperbola at \( M \). Suppose \( l \) intersects the \( x \)-axis at \( P \) and the \( y \)-axis at \( Q \).
   - The coordinates of \( P \) and \( Q \) are \( (2a, 0) \) and \( (0, 2b) \) respectively.

4. **Convexity and Tangent Line**:
   - Any point \( N = (x, y) \) of the convex figure must lie inside the triangle \( \triangle OPQ \).
   - If \( N \) lies outside \( \triangle OPQ \), the line segment \( NM \) will intersect the hyperbola again, which contradicts the convexity of the figure.
   - Therefore, the convex figure must be contained within the rectangle with vertices at \( (2a, 0) \), \( (0, 2b) \), \( (-2a, 0) \), and \( (0, -2b) \).

5. **Scaling Factor \(\lambda\)**:
   - The smallest \(\lambda\) such that the scaled rectangle covers the convex figure is \( \lambda = 2 \).
   - For a rhombus centered at \( O \), \(\lambda = 2\) is exactly the scaling factor needed to cover the figure.

The final answer is \( \boxed{ \lambda = 2 } \).