Što je više delova, to je skica važnija.

This is a very common Serbian pattern used to express a correlation:

što X, to Y = the more X, the more Y
or more generally as X increases, Y increases

So:

Što je više delova, to je skica važnija.
means something like:

The more parts there are, the more important the sketch is.

This pattern is extremely useful in Serbian. For example:

• Što više učim, to više znam. = The more I study, the more I know.
• Što je teže, to je zanimljivije. = The harder it is, the more interesting it is.

Here, što does not mean what. It is part of this fixed comparative structure.

Because više usually requires the genitive plural of countable nouns.

• deo = part
• delovi = parts (nominative plural)
• delova = of parts / parts (genitive plural form)

After quantity words like više, manje, mnogo, nekoliko, Serbian often uses the genitive:

• više delova = more parts
• mnogo ljudi = many people
• nekoliko knjiga = several books

So više delova is the normal and correct form.

Both instances of je are forms of the verb biti = to be.

The sentence has two linked clauses:

• Što je više delova
• to je skica važnija

In the first part, je means something like there are / there is in an idiomatic sense. Serbian often uses to be where English might use there is/there are.

So:

• Što je više delova = the more parts there are
• to je skica važnija = the more important the sketch is

This is normal Serbian structure.

Važnija is the comparative form of važna = important.

Basic adjective:

• masculine: važan
• feminine: važna
• neuter: važno

Comparative:

• masculine: važniji
• feminine: važnija
• neuter: važnije

Since skica is a feminine singular noun, the adjective must agree with it:

• skica → feminine singular
• therefore važnija

So:

• važnija skica or skica je važnija = the sketch is more important

Because skica is the subject of the second clause:

to je skica važnija

In Serbian, the subject is normally in the nominative case, and here skica is the thing being described as more important.

So:

• skica = nominative singular
• važnija agrees with skica

This part literally works like:

the sketch is more important

Not in this sentence.

Serbian što can have several functions depending on context. It can mean things like what, that which, or appear in other constructions. But here it is part of the fixed pattern:

što ..., to ...

So here it does not mean what.
It is better to understand the whole structure together, not to translate što by itself.

Not very naturally.

A word-for-word rendering would be something like:

As there are more parts, so the sketch is more important

But that sounds unnatural in English. The natural English equivalent is:

The more parts there are, the more important the sketch is.

So when you see što ..., to ..., it is usually best to translate the whole pattern, not each word separately.

Više means more.

It is the comparative form related to quantity, roughly corresponding to more in English.

In this sentence:

• više delova = more parts

You will see više used very often:

• više vremena = more time
• više novca = more money
• više ljudi = more people

So here it is simply marking an increase in quantity.

To some extent, yes, but this exact order is very natural.

Serbian allows more flexibility than English, but the pattern što ..., to ... is usually kept clearly recognizable. The sentence as given sounds standard and natural:

Što je više delova, to je skica važnija.

You may see small variations in other sentences, especially in spoken language, but for a learner, this is a good model to follow.

It sounds like a general statement or general principle.

The structure što ..., to ... often expresses a rule, tendency, or correlation:

• when one thing increases, another also changes

So this sentence does not sound like it is about one single moment only. It sounds more like:

In general, when there are more parts, the sketch becomes more important.

Sometimes Serbian can express similar ideas in other ways, but in this sentence the use of je is completely normal and idiomatic.

The phrase:

Što je više delova ...

naturally means:

The more parts there are ...

So although English often prefers there are, Serbian commonly uses biti in this kind of construction. As a learner, it is best to accept this as a normal Serbian pattern rather than trying to force an English-style structure onto it.