i-adj + sa

[82riceballs]

I've noticed that sometimes, after i-adjectives, there is a "sa"
e.g. 寂しさ、美しさ、おいしさ、etc.
is it the same thing as 'koto'?

Also, you know how one would say 食べなさい？ Well, how would you say
'please don't eat'?

 

[MeAndroo]

Adding the sa indicates it's a measurement. EX. takai is tall, takasa is height.

Tabenasai is not "please eat." It's "eat," a command. Please eat would be
"tabete kudasai" and please don't eat would be "tabenaide kudasai."

More info.

 

[quamp]

さ in some very informal situations can be used as a smug emphasis.
良いさ can be translated as "it's good enough!" Usually it implies that the speaker was asked a question that s/he believes that the asker already knew the answer to, and shouldn't have asked the question in the first place. This is usually used by male speakers.

 

[MeAndroo]

I should have expanded and said that you must drop the i kana at the end of the adjective while adding the sa to change the word correctly.

Sa as a casual particle as a softener is also used in a number of situations, often replacing ne or na.