Why learning SI base unit are important to understanding the sizes of a different object?

A base unit (also referred to as a fundamental unit) is a unit adopted for measurement of a base quantity. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. ... The SI is made up of 7 base units that define the 22 derived units with special names and symbols. The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. The various features and future prospects of the International System of Units (SI) are described. The SI is based on seven selected base units, corresponding to the seven quantities such as length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Units are of two types: Fundamental units. Derived units. or powering the base units in various combinations, For example:
mechanical work is force applied multiplied by distance moved and has the unit newton metre written as Nm.
speed is distance divided by time and has the unit metre per second written as ms. The three measures are descriptive, diagnostic, and predictive. Descriptive is the most basic form of measurement.

An atom that has fewer electrons that protons is called a positive ion.