Я выбрала квадратную рамку, потому что она лучше подходит к этому маленькому плакату.

Выбрала is the past tense feminine singular form of выбрать (to choose).

Russian past tense agrees with the gender of the person who did the action:

• я выбрал = I chose (said by a man)
• я выбрала = I chose (said by a woman)

So this sentence tells you that the speaker is female.

Because it is the direct object of the verb выбрала.

The speaker chose something, so that noun goes into the accusative case. For many feminine nouns ending in -а, the accusative singular changes to -у:

• рамка = frame (dictionary form, nominative)
• рамку = frame (accusative)

So:

• Я выбрала рамку = I chose a frame

The adjective must agree with the noun it describes in gender, number, and case.

Since рамку is:

• feminine
• singular
• accusative

the adjective must also be feminine singular accusative:

• квадратная рамка = a square frame
• квадратную рамку = a square frame (as the object of the verb)

So квадратную matches рамку.

Потому что means because.

It introduces the reason:

• Я выбрала квадратную рамку, потому что... = I chose the square frame, because...

Literally, Russian learners sometimes break it down as something like for that reason that, but in real usage you should simply think of it as the standard conjunction because.

In Russian, nouns have grammatical gender, and pronouns agree with that gender even for objects.

• рамка is feminine
• so the pronoun referring to it is она (she/it)

In English, we normally say it, but in Russian:

• рамка ... она ...

So она here means it, referring back to рамка.

Here подходить к means to suit, to go well with, or to match.

So:

• она лучше подходит к этому маленькому плакату = it suits this small poster better / it goes better with this small poster

This is a very common Russian pattern:

• подходить к чему-то = to suit / match something

The preposition к is required by this verb in this meaning.

Because the preposition к requires the dative case.

So all the words in that phrase change into dative singular masculine:

• этот → этому
• маленький → маленькому
• плакат → плакату

That is why you get:

• к этому маленькому плакату

This whole phrase means to/with this small poster in the sense of matching or suiting it.

Because лучше is not an adjective here. It is the comparative form meaning better.

It works like an adverb or predicative comparative and does not agree with the noun:

• она лучше подходит = it fits better / suits better

Compare:

• хороший = good
• лучше = better

So you do not change лучше for gender or case.

Yes. Выбрала comes from выбрать, which is perfective.

Perfective verbs usually present an action as:

• completed
• finished
• resulting in an outcome

So Я выбрала квадратную рамку means:

• I chose the square frame
• the choice has been made

If you used the imperfective выбирала, it would usually suggest:

• the process of choosing
• repeated choosing
• choosing in progress
• or a more contextual meaning like I was choosing

Here the completed choice is what matters, so выбрала is natural.

Because the subject is она, which is singular.

The relevant part is:

• она подходит = it suits / it fits

Even though English sometimes uses structures that feel less personal, Russian still agrees the verb with the grammatical subject:

• она подходит = feminine singular subject + singular verb

Yes, Russian word order is quite flexible, though the neutral version here is very natural:

• Я выбрала квадратную рамку, потому что она лучше подходит к этому маленькому плакату.

You could change the order for emphasis, for example:

• К этому маленькому плакату она подходит лучше.
• Квадратную рамку я выбрала, потому что...

But the original sentence is the most straightforward and neutral for learners.

Because маленький and малый are different words.

• маленький means small/little in the ordinary physical sense
• малый is more limited, more formal, or used in specific expressions

So for small poster, Russian naturally uses маленький. In the dative singular masculine, that becomes:

• маленький → маленькому

Yes, подходить can mean several things, including to approach physically.

But meaning depends on context.

Common meanings:

• подходить к дому = to approach the house
• эта рубашка тебе подходит = this shirt suits you
• эта рамка подходит к плакату = this frame goes well with the poster

In your sentence, since the subject is a frame and the object is a poster, the meaning is clearly suits / matches / goes with, not physical movement.