Learn Russian: Эта комната маленькая, а та — большая.

In Russian, а is used to show contrast or compare two different things, much like 'whereas' in English. Here we are contrasting the small room with the big one. You use и to join similar things, and но for a direct contradiction.

What is the purpose of the dash (—) between та and большая?

The dash is often used in Russian writing to replace the present tense verb 'to be' (is/are), which is omitted. Here, it stands in for the missing 'is' to make the second part of the sentence clear: 'that (one) [is] big'.

Why are the words Эта and та used here?

Эта means 'this' (something close by), while та means 'that' (something further away). Both are in the feminine form because they describe the feminine noun комната (room). If the word was masculine, like house (дом), you would use Этот and тот.

How do you translate the word 'one' from 'that one is big'?

You don't! In English, we use 'one' so we don't have to repeat the noun and say 'that room is big'. Russian doesn't use placeholder words like this. You just use the pronoun and the adjective on their own, so та — большая literally just means 'that — big'.