Scale is the measure of how big or small something is. Most characters in ORPS are going to be Scale 1 as the size of a typical human is the base of the scale. The 'average human' is considered to be 2 yards/meters tall for our purposes. While for humans this is height, the Scale system uses the longest dimension. So a tiger that is 2 meters long is also scale 1 even though it's height is half that of a human. The same would apply for something 2 meters wide just as well.
Things larger than human size or Scale 1 are measured and how many times larger they are than 2 meters. At around 12 feet high and 30 feet long an African elephant is Scale 4 or 5. This is worked out as longest dimension (length in this case) in meters (9.1) and simply dividing it by 2. The result is actually 4.55 so it is safe to say that an African elephant will be between Scale 4 and 5. If it is a single individual we can simply call it Scale 5 by rounding up.
This also works for things smaller than human scale. The way we write it simply shifts to a fraction to show how much of Scale 1 the size is. Something 3 foot long is Scale 1/2 as it is half the size of Scale 1. Something 1/3rd of a foot long is Scale 1/20.
Size and weight (and therefor mass) are usually fairly proportional. This is why a tiger that is Scale 1 and a human that is Scale 1 will weigh about the same. Because of this Scale also implies weight. However weight of an object can vary and to accommodate this you can add the prefixes 'light' and 'heavy' to the Scale. So a Cast iron safe may be Scale 1/2 and heavy so we write it as Scale 1/2 heavy. This is discussed further in the section on vehicles.