zhè jiàn yīfu bú tài héshēn, wǒ xiǎng huàn yí jiàn dà yìdiǎnr de.

件 is the measure word for many items of clothing. In Mandarin, you usually need a measure word between:

• a demonstrative and a noun: 这 + 件 + 衣服
• a number and a noun: 一 + 件 + 衣服

So 这件衣服 means this item/piece of clothing, and 一件大一点儿的 means one slightly bigger one.

Because 件 already does that job. The normal pattern is:

demonstrative + measure word + noun

So:

• 这件衣服 = this piece of clothing
• not 这衣服 in standard usage when you want to say this piece/item of clothing

Mandarin often requires this extra classifier where English does not.

不太 means not very or not quite. It softens the statement.

So:

• 不合身 = does not fit
• 不太合身 = does not fit very well / is not quite the right fit

This sounds more natural and polite in a shopping situation than a very blunt negative.

合身 means to fit well on the body. It is commonly used for clothing.

It usually refers to size and overall fit, not just whether something is physically possible to wear. So 这件衣服不太合身 means the clothes are wearable, but the fit is not quite right.

This is a tone change rule. 不 is normally fourth tone (bù), but before another fourth-tone syllable, it changes to second tone.

• 太 is fourth tone
• so 不太 is pronounced bú tài

The character stays the same; only the pronunciation changes.

一 changes tone depending on what comes after it.

Basic form: yī

But in normal speech:

• before a fourth-tone syllable, it becomes yí
• before a first, second, or third-tone syllable, it becomes yì

So here:

• 一件 → yí jiàn because 件 is fourth tone
• 一点儿 → yì diǎnr because 点 is third tone

This is a very common tone sandhi rule.

大一点儿 means a little bigger or slightly bigger.

Pattern: adjective + 一点儿

So:

• 大 = big
• 大一点儿 = a bit bigger

If you only say 大, it can sound too vague or too strong, like big rather than a little bigger. In a store, 大一点儿 is a very natural way to ask for the next size up.

The 的 replaces the noun that is understood from context.

The full idea is something like:

• 一件大一点儿的衣服

But since 衣服 was already mentioned, Mandarin often leaves it out and keeps 的:

• 一件大一点儿的 = a slightly bigger one

So here 的 turns the descriptive phrase into something like the one that is a bit bigger.

Yes. 换一件大一点儿的衣服 is also correct.

The original sentence drops 衣服 because it is already obvious from context. That makes it sound natural and efficient, much like English I want to change it for a bigger one instead of repeating clothing.

Here 换 means to exchange/switch. In a clothing-store context, it usually means changing for another item, size, or color.

So 我想换一件大一点儿的 means:

• I’d like to exchange it for a slightly bigger one
• or I want to switch to a slightly bigger one

It does not mean I want to get dressed or I want to alter the clothes.

一点儿 means a little or a bit.

The 儿 is an erhua ending, common in standard Mandarin and especially northern speech. In this phrase, 一点儿 is very common and natural.

You may also hear:

• 一点 in some accents or speaking styles
• 一点儿 in standard teaching pronunciation

Both relate to the same basic meaning: a little bit.

It follows a very common Mandarin order:

verb + quantity/classifier phrase + descriptive phrase + 的

So:

• 换 = exchange
• 一件 = one item
• 大一点儿 = slightly bigger
• 的 = the one that is...

Together: 换一件大一点儿的 = exchange for a slightly bigger one

The classifier phrase comes before the description, which may feel different from English at first.

Because the speaker is expressing a current wish or intention, not a completed action.

• 我想换一件大一点儿的 = I want to exchange it for a slightly bigger one

If 了 were added, the meaning would change depending on placement and context. Without 了, it is a straightforward statement of what the speaker wants right now.