While correct, the description from the Concepts guide does not provide a quick way to estimate the stored length, in bytes, of a numeric value. The exponent does not impact the length. The stored length for 123.45 and 1234.5 and 12345 and 123450000000 will be identical. The minimum length is 2 bytes. The maximum length for the mantissa is 20 bytes.
Negative numbers take one byte more than positive numbers.
The stored length is impacted by the number of significant digits in a number, not its number of digits. The table was then populated with numeric values from 1 to 38 significant digits of precision. Negative numbers were also tested, following the same methodology. In all cases, the value was a string of 9 s of the specified length. If the number does not end in zero, then this is the number of significant digits.