A difficult question of area and volume

I think the ideal shape is circle.
I have considered two assumptions. A square and a circle.
let me explain it.

For the square
Let l be the length of the square
4l =40
l=10
the area of the square =10x10=100cm2

For the circle
Let r be the radius of the circle
2pai r =40
r=6.3662
the area of the circle=r^2 pai =6.3662^2 pai=127.3cm2 >100 cm2
Hence, the ideal shape is circle

The ideal shape is a circle.

For the same perimeter, a circle encloses the largest area.

Hence, the ideal shape Peter wants is a circle with radius
= 40 / (2Pi)
= 6.37 cm (to 3 sig. fig.)