Zhǐyǒu xiān shìshi, cái zhīdào zhège chǐmǎ shìbushì héshēn.

只有...才... is a very common pattern meaning only if... then... or only by... can....

So:

• 只有先试试 = only by trying it first
• 才知道这个尺码是不是合身 = can you know whether this size fits

It shows a necessary condition. The speaker is saying that trying it first is the thing you must do before you can know.

These two patterns are related, but they are not the same.

• 只有...才... = only if... then...
  • emphasizes a necessary condition
• 只要...就... = as long as... then...
  • emphasizes a sufficient condition

Here, 只有先试试，才知道... means:

• Trying it first is the only way to know

If you said 只要先试试，就知道..., it would sound more like:

• As long as you try it first, you’ll know

That is possible in some contexts, but 只有...才... is stronger and fits the meaning better here.

先 means first or beforehand.

In 只有先试试，才知道..., it shows order:

• first, try it
• after that, you can know whether it fits

So 先 is not just about time; it helps express the logic that the trying must happen before the knowing.

试试 is the reduplicated form of 试. This often makes the verb sound:

• lighter
• more casual
• less forceful

So 试试 is like:

• try it a bit
• give it a try

It sounds more natural in everyday speech than a plain 试 in this kind of sentence.

You can also say 试一试, which means basically the same thing.
试试 is just a shorter, very common spoken form.

In this sentence, it really means try on.

Because the sentence talks about:

• 尺码 = size
• 合身 = fitting the body well

the context is clearly clothing, shoes, or something wearable.

Mandarin often leaves some information unstated when it is obvious from context, so 试试 here naturally means:

• try it on
• more explicitly, 试穿一下

A fuller version could be:

• 只有先试穿一下，才知道这个尺码是不是合身。

The object is omitted because it is understood from context.

In English, you might want to say:

• try this shirt on
• try this size on

In Mandarin, if everyone already knows what item is being discussed, it is very common to leave the object out.

So here, the omitted object could be understood as something like:

• the clothes
• this item
• this size

Mandarin often drops things that are obvious in the situation.

才 means only then in this pattern.

It marks the result that happens only after the condition is met:

• 只有先试试，才知道...
• Only after trying it first do you know...

So 才 is important. It matches 只有 and completes the structure.

Without 才, the sentence would sound incomplete or much less natural with this pattern.

Here, 是不是 means whether or not.

It is not the main sentence question. Instead, it introduces an embedded yes/no question after 知道:

• 知道这个尺码是不是合身
• know whether this size fits

So the whole sentence is a statement, not a direct question.

A similar English pattern is:

• I know whether it fits
• I don’t know whether it fits

In Mandarin, this kind of embedded question is very common.

Yes. A very common alternative is:

• 合不合身

So you could also say:

• 只有先试试，才知道这个尺码合不合身。

Both mean whether this size fits.

The difference is mostly stylistic:

• 是不是合身 sounds a little more explicit
• 合不合身 uses the standard A-not-A pattern directly on the adjective phrase

Both are natural.

合身 specifically means that clothing fits the body well.

So it is used for things like:

• clothes
• shoes
• wearable items

Examples:

• 这件衣服很合身。 = This piece of clothing fits well.
• 裤子不太合身。 = The pants don’t fit very well.

合适 is broader. It means suitable, appropriate, or a good fit in a general sense.

So:

• 合身 = fits your body
• 合适 = suitable in general

In this sentence, 合身 is the better word because the topic is size and how something fits when worn.

In Mandarin, when you use 这 or 那 before a noun, you usually need a measure word / classifier:

• 这 + measure word + noun

So:

• 这个尺码 = this size

Here, 个 is the classifier.

English does not usually need anything like this, but Mandarin does. That is why you cannot normally just say 这尺码 in standard everyday speech.

Yes. The subject is omitted, and it is understood from context.

Mandarin often drops the subject when it is obvious. In this sentence, the subject could be understood as:

• you
• we
• one / someone

In a shop, it most naturally means you:

• Only by trying it on first can you know whether this size fits.

Mandarin often puts the condition first and the result after it.

So the structure is:

• 只有先试试 = only by trying first
• 才知道这个尺码是不是合身 = only then know whether this size fits

A very literal breakdown would be:

• only have-to first try-try, only-then know this size whether fit-body

That sounds unnatural in English, but it is normal Chinese word order.

Natural English translations would be:

• Only by trying it on first can you know whether this size fits.
• You can only know whether this size fits if you try it on first.