Anotaciones en español al final

SO + ADJECTIVE/ADVERB

We use this to make exclamations or to express a superlative idea (that the quality of the adjective/adverb is present in a very big degree).

Exclamation: So big = wow, It's incredible !
You're so beautiful !
That tree is so tall !
This song is so cool, I love it !
Where were you? It's so late !

High degree: so big = very very big.
I like her because she's so nice
The box was so heavy that we had to call some friends to help us move it
Why do you talk so slowly?
 
It can also be used as an emphasizer with: much, many, few, little

Thanks, that's so much better
There were so many people that we couldn't enter
But that's so little !

SUCH  A + (ADJECTIVE) + NOUN
 
It expresses the same idea as SO, but it is used with nouns (with or without a companion adjective)
 
The house is so big = It's such a big house
 
I've never met such a nice person
You'll never see such a beautiful flower again
You've got such strange friends

When we use SUCH for exclamations, we can use SUCH + Noun, but it is much more common to use SUCH + Adjective + Noun:

She is such a girl (correct, but not common)
She is such a nice girl (correct and common)

Such or Such a ?

We can only use the article A with countable singular nouns, so that is the rule, if you can use A you use it, if you can't, well, you don't:

a nice house - such a nice house (cont. sg)
a big tree - such a big tree  (cont. sg)
beautiful flowers - such beautiful flowers  (cont. pl.)
some pretty girls - such pretty girls (cont. pl.)
good wine - such good wine  (uncont.)
bad weather - such bad weather  (uncont.)

For Spanish Speakers

SO / SUCH equivalen en español a TAN (exclamativo) o a -ÍSIMO (superlativo), pero las exclamaciones se pueden traducir de muchas maneras según la frase. Veamos ejemplos:

Exclamación: - you're so beautiful! = ¡eres tan bonita!, ¡qué bonita eres!
Superlativo: - I like her, she's so beautiful = me gusta, es guapísima / es preciosa

Wow, that's so big! = ¡caray, es enorme!
He's so patient with her = tiene una paciencia enorme con ella / ¡ tiene más paciencia con ella... !
why do you talk so slowly? = ¿por qué hablas tan despacio?
It's such a big house = es una casa grandísima
That's such a nice horse!= ¡vaya caballo! /¡ menudo caballo que está hecho!
you've got such strange friends = tienes unos amigos rarísimos (=tan raros!)
Your eyes are so big! = qué ojos más grandes tienes

¿Cómo saber cuándo "so big" significa "tan grande" y cuándo significa "grandísimo"?

La respuesta es que no hace falta saberlo, son simplemente dos formas de traducir la misma idea, así que si tienes que traducir, utiliza en español la forma que te suene más natural. Al hablar sí podemos notar una diferencia en la entonación, la misma que en español, cuando hacen entonación de exclamación, ascendente, equivale a nuestro "tan", y si hacen entonación enunciativa, descendente, equivale a nuestro "-ísimo", pero en el fondo viene a ser la misma cosa, solo es cuestión de énfasis.