Stůl je dva metry dlouhý.

They usually do when they directly modify a noun: dlouhý stůl = a long table.

But in Stůl je dva metry dlouhý, dlouhý is not directly attached to the noun. It is a predicate adjective after je (is), so the structure is more like:

• Stůl = the table
• je = is
• dva metry dlouhý = two meters long

So this is not long table, but the table is long.

Because the adjective agrees with stůl.

Stůl is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative: dlouhý.

Compare:

• lavice je dva metry dlouhá = the bench is two meters long
• okno je dva metry dlouhé = the window is two meters long

It is nominative, because it is the subject of the sentence.

• Stůl = the table
• je = is

So stůl is the thing being described.

Because after 2, 3, 4, Czech normally uses the plural form of the noun:

• dva metry
• tři metry
• čtyři metry

After 5 and higher, Czech usually uses the genitive plural:

• pět metrů
• šest metrů
• deset metrů

So:

• dva metry dlouhý
• but pět metrů dlouhý

Because metr is a masculine noun.

The forms are:

• dva for masculine and neuter nouns
• dvě for feminine nouns

So:

• dva metry = two meters
• dvě minuty = two minutes

For a learner, the most useful explanation is: this is a measure expression meaning extent or dimension, and Czech commonly uses this like an accusative of extent.

In practice, with metry, the form metry looks the same in nominative and accusative plural, so you do not see a visible difference here.

The important thing to remember is simply:

• Czech often uses a bare number + unit to express size, length, height, distance, duration, etc.
• No extra preposition is needed.

Because Czech normally does not need one here.

Czech commonly says dimensions like this:

• je dva metry dlouhý = is two meters long
• je tři metry široký = is three meters wide
• je deset metrů vysoký = is ten meters tall/high

If you want, you can also use a longer expression such as:

• má dva metry na délku = it has two meters in length

But in everyday Czech, the shorter structure is very normal.

Yes, in a normal sentence like this, je is necessary.

Czech uses být (to be) with predicate adjectives:

• Stůl je dlouhý. = The table is long.
• Stůl je dva metry dlouhý. = The table is two meters long.

You may sometimes see je omitted in headlines, notes, or very informal styles, but not in standard full sentences like this.

Yes. That is also possible.

Czech word order is flexible, and both versions are understandable:

• Stůl je dva metry dlouhý.
• Stůl je dlouhý dva metry.

The meaning is the same. The difference is mainly one of emphasis and style. The version you were given is a very common and natural way to say it.

Yes. A very natural alternative is:

• Stůl měří dva metry.

That means The table measures two meters.

If you want to make it extra clear that you mean length, you can say:

• Stůl měří dva metry na délku.

So both patterns are good:

• Stůl je dva metry dlouhý.
• Stůl měří dva metry.

Yes. The adjective dlouhý agrees with stůl, not with metry.

That is an important point.

The core description is:

• stůl je dlouhý = the table is long

The phrase dva metry just tells you how long.

So the agreement stays with stůl:

• stůl ... dlouhý
• not with metry

The adjective still agrees with stůl, so dlouhý stays the same. Only the number phrase changes:

• Stůl je jeden metr dlouhý.
• Stůl je dva metry dlouhý.
• Stůl je tři metry dlouhý.
• Stůl je čtyři metry dlouhý.
• Stůl je pět metrů dlouhý.

Notice the change:

• 2–4: metry
• 5+: metrů

Yes. Because of dlouhý together with metry, it clearly refers to physical length.

Czech dlouhý can mean long in different senses depending on context, but with measurements like metry, it means actual spatial length.

So Stůl je dva metry dlouhý can only mean that the table has a length of two meters.