Learn English – Difference between “magnitude” and “multitude”

Usually, size connotes physical dimensions while magnitude connotes either a numerical measure (particularly a non-linear one) of any sort of amount or metaphorical size.

"What size is that screw?" (Physical dimensions)

"A problem of immense magnitude" (Metaphorical size)

You can also use size to mean metaphorical size, but you usually can't use magnitude alone to mean physical size. (Nobody would say "What's your sneaker magnitude?")

First and foremost, the words constitution and structure are considered synonyms.

That being said, I would use them slightly differently in my own speech and writings. I would choose the word constitution when talking about what something is made of, while I would use structure to talk about how it was made. This may be a difference from others, or a regional distinction. The definition of structure supports my own definition; the word structure is used to describe:

mode of building, construction, or organization; arrangement of parts, elements, or constituents

Likewise, the two main definitions of constitution are as follows:

1. the way in which a thing is composed or made up; makeup; composition
2. the physical character of the body as to strength, health, etc.

Therefore, I would conclude that both constitution and structure can be used to describe how something is made. Structure tends to refer more to the arrangement of the entities the object is composed of, while constitution refers to the identity of the entities themselves.