What is the difference between the use of le/la meme and pareil?

Bonjour Beverley !

That's an interesting question.
The pronouns "Le/la même" refer to a previously mentioned thing = the same *one*
whereas in this context (c'est pareil), "pareil"is actually acting as an adverb = samey/samely

As an expression, "C'est pareil" is a more of a general statement, literally meaning "It's samey", which translates as "It's the same thing."

I hope that's helpful!
À bientôt !

Yes, but two more questions:-
1. Is there any context in which they are interchangeable?

2. When I buy fish, I usually buy two, two fish or two fillets and I need them to be the same size. When I want to ask for two the same size, I usually say "Deux pareil, SVP" Is that incorrect? Should I be saying "Deux la meme taille, SVP"?

Merci bien!!

In this case Beverley, you're using the adjective "pareil/le" meaning "identical/same", so when you say "Deux pareils, svp", you're literally saying "Two identical, please", which is indeed the colloquial way to ask this in French :)