If the scale factor is m:n = m/n
If the length is x of the original shape, the dimensions on diagram are

If the width is y of the original shape, the dimensions on diagram are

Here the scale factor is 1/40
The dimensions of the arena hall are 200 m by 85 m.
Hence length = 200m
So length on diagram
=

On diagram it shall be 5 m
The width = 85 m
So width on the diagram
=

On diagram width shall be 2.125m
The length on the diagram shall be 5 m & the width shall be 2.125 m.