El desnivel es tan grande como el de la cuesta.

Because desnivel is a masculine singular noun, so it takes the masculine singular definite article el.

• el desnivel = the difference in level / the slope / the incline
• The article is used here because the sentence is talking about a specific incline or level difference, not just any one in general.

Desnivel usually refers to a difference in height or level between two points. Depending on context, it can mean:

• difference in elevation
• drop
• incline
• gradient

In this sentence, it refers to the steepness or level difference being discussed.

Tan ... como is the structure used to say as ... as with adjectives or adverbs.

Here:

• tan grande como = as big/large as

So the pattern is:

• tan + adjective + como

Examples:

• Es tan alto como su hermano. = He is as tall as his brother.
• Es tan difícil como parece. = It is as difficult as it seems.

In your sentence:

• El desnivel es tan grande como... = The incline/elevation difference is as great as...

Because tan is used before adjectives and adverbs, while tanto is used with nouns or as a pronoun/adverb in other structures.

Here grande is an adjective, so you need:

• tan grande = as big not
• tanto grande ❌

Compare:

• tan grande como = as big as
• tanto desnivel como = as much difference in level as

That el stands in for the noun desnivel, which is being omitted to avoid repetition.

The full version would be:

• El desnivel es tan grande como el desnivel de la cuesta.

But Spanish normally avoids repeating the noun when it is obvious, so it becomes:

• El desnivel es tan grande como el de la cuesta.

So el de la cuesta means something like:

• the one of the slope
• more natural English: the one on the slope / that of the slope

Because Spanish often drops a repeated noun when the article makes it clear what is meant.

This is very common:

• Mi coche es más rápido que el de Ana. = My car is faster than Ana’s.

  • Here el = el coche

• La casa roja es más grande que la azul. = The red house is bigger than the blue one.

  • Here la = la casa

So in your sentence, el replaces el desnivel.

Literally, de la cuesta means of the slope/hill/street going uphill or downhill.

• cuesta can mean slope, hill, or steep street/road
• de often means of, but in English the most natural translation may be:
  • of the slope
  • on the slope
  • of the hill

The exact wording depends on context, but grammatically it is a de + noun phrase modifying the omitted noun desnivel.

Because cuesta is a feminine noun:

• la cuesta = the slope / hill

Even though many nouns ending in -a are feminine, it is still something you learn noun by noun. Here it follows the usual pattern.

Yes, grande here means great/large in the sense of degree or extent, not physical size in a simple everyday sense.

So:

• un desnivel grande = a large/great difference in level

You could also use other words in different contexts:

• empinado = steep
• pronunciado = pronounced/marked
• fuerte = steep/strong in some contexts

But grande works naturally with desnivel because you are talking about how big the elevation difference is.

The sentence breaks down like this:

• El desnivel = subject
• es = verb
• tan grande como el de la cuesta = comparison

So the structure is:

• [subject] + ser + tan + adjective + como + comparison

In full:

• El desnivel = the elevation difference
• es = is
• tan grande como = as great as
• el de la cuesta = the one belonging to / on the slope

Yes. A fuller version would be:

• El desnivel es tan grande como el desnivel de la cuesta.

A slightly different but still natural version might be:

• El desnivel es igual de grande que el de la cuesta.

Both express a comparison. The original sentence is more elegant because it avoids repeating desnivel.

In standard Peninsular Spanish, a rough pronunciation would be:

• El desnivel es tan grande como el de la cuesta
• /el dezniˈβel es taŋ ˈgɾande ˈkomo el de la ˈkwesta/

A few helpful points:

• desnivel: the s before n may sound a bit like z
• grande: the g is a hard g sound
• cuesta: cue sounds like kwe

In much of Spain, the c in words like cima or cena is pronounced like th, but here cuesta has cu, so it keeps a k sound.

Yes, it is grammatical and natural, especially in descriptive, geographical, technical, or formal contexts.

It may sound a bit more specific or descriptive than everyday casual speech because desnivel is a somewhat precise word. In conversation, depending on context, a speaker might also say something simpler like:

• La pendiente es tan grande como la de la cuesta.
• Tiene la misma inclinación que la cuesta.

But your original sentence is perfectly correct and natural.