Learn Portuguese (Portugal): A malha está a alargar-se com cada lavagem.

The verb alargar is naturally transitive, meaning it is used when someone stretches or widens something else (like a person stretching a sweater). By adding the anti-causative -se, we remove the external doer. This shifts the focus to the fabric itself, meaning "the knit is stretching on its own."

Could the pronoun -se go anywhere else in this sentence?

In European Portuguese, when using the present continuous structure (estar a + infinitive), the pronoun strongly prefers to attach to the end of the infinitive with a hyphen (alargar-se). Placing it before the verb is common in Brazilian Portuguese, but sounds unnatural in Portugal for this specific structure.

Does alargar always translate to "to stretch"?

Not exactly. Its core meaning is "to widen" or "to make larger" (it comes from the adjective largo, meaning wide). While it translates perfectly to "stretch" when talking about clothes losing their shape, you will also see it used for widening a road (alargar a estrada) or broadening your knowledge.

Why use cada lavagem (every wash) instead of todas as lavagens (all washes)?

Using cada lavagem highlights the progressive, step-by-step nature of the fabric stretching. It pairs perfectly with the continuous action of está a alargar-se to show that the physical change happens a little bit more every single time it goes through the laundry.