What is the purpose of the suffix “さ” on adjectives?

Question

What is the purpose of the suffix "さ" on adjectives like 美しさ and 多さ?

The former is the title of an essay by Banana Yoshimoto, so I don't have much context for it. The book editors translated it as "On beauty" which didn't help me understand what this さ business was all about.

In the case of the latter, she writes:

"品揃えの多さについつい買い込んでしまい、大荷物を抱えて併設の花屋に行った。"

Which I understand as:

"There were many goods there that, against my better judgement, I had purchased. Carrying the bags I went to the attached flower shop."

Answer 1

It serves the same purpose as "-ness" in English; making an adjective into a noun.

速さ = speed; quickness
高さ = height; "tallness"
長さ　= length; "longness"
新しさ　= newness
大きさ = size; largeness

and in your case:

多さ = multitude 美しさ = beauty

So in this context, the expression means "I, against my better judgement, bought a multitude of goods."

Answer 2

As phoenixheart6 wrote, suffix -さ makes a noun from an adjective. I am not sure if you have a problem with this. But it seems that you have a misunderstanding about the role of に. In a sentence

品揃えの多さについつい買い込んでしまった。

に means “because of.” 買い込む means “to buy many things.” Therefore, it means:

Because of the variety of goods they sell, I ended up buying too many things against my better judgment.