If someone asks "How far is Brazil from Argentina", a possible answer is "It's about X kilometers". However, if someone asks "How high is this mountain?" you can answer "It's about X meters high". I think that "It's about X kilometers far" would sound weird, but I don't whether it's correct or not. Could you help me?
In sentences like
- How high is the mountain?
- How far is Brazil?
high and far function as adjectives, and the question asks about a quality of the adjective.
High can be used on its own, intensified, or qualified:
- The mountain is high.
- The mountain is very high.
- The mountain is three miles high.
Far can only be intensified. It can't be qualified, and it's odd on its own:
- ?Brazil is far.
- *Brazil is 12000 miles far.
- Brazil is very far.
The difference between far and high may be due to direction of measurement. Although we live in a three-dimensional world with mountains, we move on its surface and default to measuring in its two dimensions. High [and deep] can be qualified because when we measure vertically we need to specify that direction; far can't be qualified because it doesn't refer, and doesn't need to refer, to a direction.
If you need to qualify far, you need to use a word which can be qualified like away ("Brazil is 12000 miles away") but that's a different question. Away can't be used on its own in this sense (*Brazil is away) or intensified (*Brazil is very away).
When you are discussing distance (from A to B), you have got to specify A and B, either implicitly or explicitly. In other words, distance involves two points that need to be specified.
On the other hand, height is a characteristic of a single object, so you only need to specify the object itself, again either implicitly or explicitly. So you can properly say:
A is x metres high
With distance you can use distant similarly to high, but because you need to specify A and B, the phrasing is usually slightly different, e.g.:
A is x km distant from B
A is x km distant [from here]
But it's often easier and clearer to say:
The distance from A to B is x km.