(Example assumes using an Engineering Scale, not an Architect Scale.)
Scale is written as one inch on the paper equals number of units you're working with.
Example -
1" = 40 KM
One inch on the paper equals forty kilometers on the object.
Complete example of how to choose scale.
Our drawing will be on an 8.5" x 11" piece of paper. We want 1/2" of border on our paper, so our drawing has to fit in 7.5" x 10".
Our ship is 950' long by 620' wide at its biggest dimensions.
950' / 10" = 95 This scale of 1" = 95' would completely fill page lengthwise.
620' / 7.5" = 83 This scale of 1"= 83' would completely fill page widthwise.
The bigger the number on the right, the smaller the drawing will be.
If we choose the 83 the width would fill the paper, but then the length would be too big, so the bigger number of the two scales drives which one we choose.
We don't have a 95 on our Engineering Scale, so we choose the closest number on the scale bigger than 95. Remember that we can add or subtract zeros to the scale. 95 is close to 100, and we have a 10 scale that we can say is 100, so we choose that.
1" = 100' One inch on the scale equals 100' of the object.
Another example -
Same paper as above, drawing fits in 7.5" x 10".
Our thing is 34 meters x 28 meters.
34 meters / 10" = 3.4 or 1" = 3.4 meters.
28 meters / 7.5" = 3.7 or 1" = 3.7 meters.
1" = 3.4 meters makes a bigger drawing than 1=3.7, so we start with 1=3.7.
Scales on the Engineering scale are 10, 20, 30, 40, 50, 60.
We can take away a zero to get scales of 1, 2, 3, 4, 5, 6.
4 is the closest number bigger than 3.7.
1" = 4 meters is the scale we choose.