Мірки талії готові.

Because мірки means measurements or measurements taken for fitting/sewing, and that idea is usually plural in Ukrainian, just as measurements is often plural in English.

• мірка = a single measurement
• мірки = measurements / set of measurements

In this sentence, it means the waist measurements as a set are ready.

Because талії is in the genitive singular.

The phrase мірки талії literally means measurements of the waist. In Ukrainian, this kind of noun + “of” noun relationship is usually shown by putting the second noun into the genitive case.

So:

• талія = waist
• талії = of the waist

This is very common in Ukrainian:

• колір сукні = the color of the dress
• розмір рукава = the size of the sleeve
• мірки талії = the measurements of the waist

Готові agrees with мірки.

Since мірки is:

• plural
• nominative

the adjective/predicative adjective also appears in the plural nominative form:

• готовий = ready (masculine singular)
• готова = ready (feminine singular)
• готове = ready (neuter singular)
• готові = ready (plural)

So Мірки талії готові literally means The waist measurements are ready.

In Ukrainian, the present-tense verb to be is usually omitted in simple sentences like this.

So where English says:

• The waist measurements are ready

Ukrainian naturally says:

• Мірки талії готові

There is no need to add a present-tense є here. Using є would usually sound unnecessary or marked in a basic statement like this.

Most likely sewing, tailoring, or fitting.

Мірки is especially common when talking about measurements taken for:

• making clothes
• altering clothes
• tailoring
• dressmaking

So Мірки талії готові would sound natural in a context like:

• a tailor has finished taking the waist measurements
• a fitting chart is complete
• someone is preparing clothing measurements

Yes. Ukrainian word order is flexible, although the most neutral version here is:

• Мірки талії готові.

You could also hear:

• Готові мірки талії.

But that may sound more context-dependent or stylistically marked. For a simple statement, Мірки талії готові is the safest and most natural order.

Yes, but the meaning changes slightly.

• Мірка талії готова = the waist measurement is ready
• Мірки талії готові = the waist measurements are ready

Use the singular if you mean one specific measurement. Use the plural if you mean the full set of waist measurements or multiple measurements related to the waist.

It is an adjective used predicatively.

In other words, it describes the subject (мірки) after an implied are:

• Мірки талії готові = The waist measurements are ready

This is similar to English The measurements are ready, where ready is an adjective, not a main action verb.

It could be understood, but мірки талії is more straightforward and natural for measurements of the waist.

Compare:

• мірки талії = measurements of the waist
• талійні мірки = waist-related measurements

The second version sounds more descriptive and less standard in many contexts. A learner should prefer мірки талії.

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

You can see that in the structure:

• Мірки = subject
• талії = dependent genitive noun
• готові = predicate adjective agreeing with the subject

So the grammar is:

[Subject in nominative] + [dependent noun in genitive] + [predicate adjective]

That pattern is very common in Ukrainian.