You cannot.
First, rectangular is an adjective and there is no associated noun so the question does not specify whether it is about a rectangular pyramid, a rectangular bipyramid, a rectangular prism or some other rectangular figure.
Second, in order to find the weight of an object, you need the volume of the shape, the density of its material and the local force of gravity. The volume of the shape requires length and width as well as the height so the available information is hopelessly inadequate.
Density is weight divided by volume. In the case of a rectangular solid, volume is length times width times height.
It is length * width * height.
It is 2*(Length*Breadth + Breadth*Height + Height*Length).
length x width x height
Height x Weight x Height
Height x Weight x Height
Height x Weight x Height
You multiply its density by its volume.
Volume = Length * Width * Height.
It is length * breadth * height.
It is length*width*height - in suitable measurement units.
One cannot calculate the BMI with weight alone as the height is also needed. To calculate the BMI that way you take your weight in kilograms and divide it by the square of your height in meters.