Se esta não couber, vou experimentar aquela.

Because after se when you are talking about a future possibility, Portuguese often uses the future subjunctive.

• Se esta não couber... = If this one doesn’t fit...
• The speaker is not describing a fact now; they are talking about a possible situation in the future.

The verb is caber (to fit), and its future subjunctive form is:

• quando eu couber
• se ela couber

So couber here is completely normal grammar.

Here, caber means to fit.

In this context, it usually means:

• a piece of clothing fits your body
• there is enough room for something

So:

• Se esta não couber... = If this one doesn’t fit...

With clothes, Brazilian Portuguese may also use other verbs such as:

• servir = to fit / suit
• ficar bem = to look good / fit well

But caber is perfectly possible, especially when the idea is physical fit.

They are feminine because they refer to an implied feminine noun.

For example, the hidden noun could be something like:

• a blusa = the blouse
• a saia = the skirt
• a calça = the pants
• a peça = the item

So instead of repeating the noun, Portuguese just uses the demonstrative pronoun:

• esta = this one
• aquela = that one

If the implied noun were masculine, you would use:

• este
• aquele

Example:

• Se este não couber, vou experimentar aquele.

They are both demonstratives, but they point to things at different distances.

• esta = this one
  usually something closer to the speaker
• aquela = that one over there
  something farther away from both speaker and listener, or psychologically more distant

So the sentence creates a contrast:

• this one
• that one

In everyday Brazilian Portuguese, the distinction between este/esta and esse/essa is often weaker than in formal grammar. Many speakers would naturally say essa where grammar books might prefer esta.

Because here they are being used as pronouns, not just adjectives.

Compare:

• esta blusa = this blouse
  (esta modifies a noun)
• esta = this one
  (esta replaces the noun)

So in the sentence:

• Se esta não couber, vou experimentar aquela.

the nouns are omitted because they are already understood from the context.

Because in Brazilian Portuguese, ir + infinitive is very common for talking about the future.

• vou experimentar = I’m going to try
• more formal/literary: experimentarei

Both are correct, but vou experimentar sounds much more natural in everyday speech.

So this sentence is very conversational and normal.

Yes. In this context, experimentar means to try on or to try out.

That can be confusing for English speakers because experiment in English usually means a scientific test. But in Portuguese:

• experimentar uma roupa = to try on a piece of clothing
• experimentar uma comida = to try some food
• experimentar um produto = to try a product

For clothes, Brazilian Portuguese also commonly uses:

• provar

So both of these can work:

• vou experimentar aquela
• vou provar aquela

depending on region and personal preference.

Because in Portuguese, não normally goes before the verb it negates.

So:

• caber = to fit
• não caber / não couber = not to fit

In the sentence:

• Se esta não couber...

não is negating couber, so the meaning is if this one doesn’t fit.

Yes, especially in everyday Brazilian Portuguese.

Strict traditional grammar often distinguishes:

• esta = this, near the speaker
• essa = that, near the listener
• aquela = that over there, farther away

But in real Brazilian usage, esse/essa often replaces este/esta in many situations.

So:

• Se esta não couber... sounds correct and a bit more careful or formal
• Se essa não couber... is also very natural in spoken Brazilian Portuguese

Yes. It is a complete conditional sentence.

It has:

1. a condition
   
   • Se esta não couber = If this one doesn’t fit
2. a result
   
   • vou experimentar aquela = I’ll try that one

This structure is very common:

• Se chover, vou ficar em casa.
  If it rains, I’m going to stay home.
• Se não der certo, tentamos outra coisa.
  If it doesn’t work, we’ll try something else.

Yes. Caber is an irregular verb.

Its future subjunctive is based on the preterite third-person plural form:

• preterite: couberam
• stem used for future subjunctive: couber-

That gives forms like:

• se eu couber
• se você couber
• se ela couber

This pattern happens with several irregular verbs in Portuguese. So couber is not random; it follows an important conjugation pattern.

Yes, very natural.

It sounds like someone choosing between items, especially clothes:

• Se esta não couber, vou experimentar aquela.

A Brazilian speaker might say this when trying on dresses, skirts, blouses, or other items.

Very natural alternatives would be:

• Se esta não servir, vou experimentar aquela.
• Se esta não ficar boa, vou experimentar aquela.
• Se esta não der, vou experimentar aquela.
  more informal

So the original sentence is absolutely plausible in real life.